Simon Marketing 12580 Simon White Laser Labels MPN, UPC, 1' Width x 2.62' Length - 30/Sheet - 3000 Label - White Printer Supplies, Labels and Laminates.

What twin studies have thus far contributed to our understanding of individual differences in intelligence? Topics include recent evidence from studies of twin-singleton differences, twins reared apart and together, virtual twins, longitudinal analyses, prenatal environments, parenting practices, shared environments, epigenetic processes, the heritability of relevant endophenotypes, associations between genetic variance and socioeconomic status, and the search for specific genes underlying intelligence.,, Nancy L. Segal and Wendy Johnson Source. Introduction Twin studies are a vital source of information about genetic and environmental influences on general mental ability.

The classic twin design —comparison of the relative similarity between monozygotic (MZ) and dizygotic (DZ) twins— is a simple and elegant approach to estimating the effects of genes and experience on developmental traits. However, while this method was considered state of the art in behavioral genetics in the 1960s and 1970s, it is now only one of many more sensitive and sophisticated twin designs.

Twin research on behavioral and medical traits, in general, and on intelligence, in particular, has advanced at an impressive rate. The focus of the present chapter is on what twin studies have thus far contributed to our understanding of individual differences in intelligence. The chapter begins by briefly summarizing key events and controversies that have marked the ontogeny of twin research on intellectual development. Subsequent sections examine new twin research designs, analytic methods, findings, and their implications. Topics include recent evidence from studies of twin-singleton differences, twins reared apart and together, virtual twins, longitudinal analyses, prenatal environments, parenting practices, shared environments, epigenetic processes (e.g., DNA methylation), the heritability of relevant endophenotypes, associations between genetic variance and socioeconomic status (SES), and the search for specific genes underlying intelligence. Links between twin studies and other research areas, both within and outside behavioral genetics, are explored.,, Fullerton, CA 92834, USA e-mail. Key Events and Controversies: A Brief Summary Twin studies began with Sir Francis Galton’s (1875) paper, “The history of twins, as a criterion of the relative powers of nature and nurture.” This monograph set forth the simple, but elegant logic that lends twins their vast research potential: “It is, that their history affords means of distinguishing between the effects of tendencies received at birth, and of those that were imposed by the circumstances of their after lives; in other words, between the effects of nature and nurture” (p.

At that time the biological bases of twinning had not been elaborated, but Galton correctly surmised that there were two types of twins: those who shared all their heredity (identical twins) and those who shared some of their heredity (fraternal twins). He concluded that greater resemblance between the former, compared with the latter, demonstrated genetic influence on the physical and behavioral traits in question.

Galton’s paper presented qualitative comparisons between twins, based on material gathered from correspondence sent to him by twins and family members. As such, it departed in significant ways from current quantitative analyses conducted with systematically recruited twin pairs. The biological bases of twinning were not revealed until the early 1900s.

Weinberg (1901) developed the formula for estimating the frequency of one-egg and two-egg twins. Newman and Patterson (1910) discovered how identical quadruplets are produced by armadillos, establishing that identical twinning occurs in mammals. (Mammalian twinning is rare, probably due to the reduced genetic variability among multiple offspring.) The natural identical twinning rate in humans is about 1/250; see Segal, 2000a.

Arey (1922) offered the terms monozygotic (MZ) and dizygotic (DZ) as labels for one-egg (identical) and two-egg twins (fraternal), respectively. Further developments in the biology of twinning have revealed numerous subclasses of both MZ and DZ twins, based on placentation and other factors (see Machin & Keith, 1999 for a comprehensive review). Organizing twin samples according to placental structure and other features has been informative with respect to some traits, including intelligence, as will be demonstrated. An early twin study by Thorndike (1905) classified twins according to age, rather than twin type, showing that cognitive resemblance did not differ across younger and older pairs.

It was not until Merriman’s (1924) investigation that the first modern twin study of intelligence appeared. [Note that the first classical MZ–DZ twin comparison was Jablonski’s (1992) study of refractive error.] Merriman found greater IQ similarity in MZ than DZ twin children, demonstrating genetic effects. Since then, numerous studies using the classic twin design, as well as variations of that design, have produced evidence consistent with Merriman’s report. However, despite the agreement across studies and the credibility generally accorded them, twin research assumptions and findings have been attacked, as well as embraced, over the years. Some of the charges against twin studies have also been raised against family and adoption studies.

For example, evidence of genetic effects on behavior (regardless of the source) has been rejected by those who mistakenly equate genetic effects with biological determinism. The results of twin studies have never indicated biological determinism. All behavioral phenotypes are products of both genes and environments. Genes do not operate in isolation, but are expressed within (or transact with) environments, at both prenatal and/or postnatal levels.

Gene–environment interactions (G × E) refer more specifically to the different expression of different genes in a given environment. For example, a high-IQ child might excel in a classroom rich with learning opportunities, whereas an average-IQ child might be less inspired. G × E also refers to the different expression of a certain genotype depending upon environmental events.

For example, a mathematically gifted child will probably display his or her quantitative skills if provided with an appropriate curriculum. However, this same child may not perform to the same degree if he or she is not sufficiently challenged. An excellent design for demonstrating such effects is co-twins control. This involves systematically exposing MZ co-twins to different experiences and assessing the outcome. For example, the effects of extra training in verbal skill could be examined by providing one twin, but not the other, with supplementary classes.

Alternatively, different training programs could be administered to each co-twin. Interactions among genes at different loci also affect behavioral phenotypes. Research shows that genetic effects underlie most measured behaviors and predispositions that are sensitive to environmental influence; however, the extent to which behaviors may be modified by environments and by experience is trait specific.

Environmental influences on all behaviors are evident by the fact that no twin study has ever reported a perfect MZ twin correlation for any measured trait. Measurement error and variable gene expression also account, in part, for MZ intraclass correlations of less than 1.00.

Other charges have been specific to twin studies. Critics have questioned the applicability of the equal environments assumption (EEA), the fundamental principle underlying the twin design. The idea here is that environmental influences on specific traits must be the same for MZ and DZ twins if findings are to be valid and generalizable.

Some people have argued that MZ twins are treated more alike than DZ twins, thus violating this assumption. This environmental challenge has been evaluated by behavioral genetic investigators and has been found wanting (Bouchard & McGue, 1993; LaBuda, Svikis, & Pickens, 1997).

Specifically, it has been found that twins who are treated more alike do not show greater behavioral resemblance than those treated less alike. For example, twins who are dressed alike do not resemble one another in personality more than twins who are dressed differently. It is important to note that if MZ twins are treated more alike than DZ twins, it is most likely associated with their genetically based behavioral similarities. Interestingly, parents who are mistaken about their twins’ zygosity tend to treat them or rate them in accordance with their true zygosity (see Segal, 2000a).

Critics have also questioned whether twins’ unique prenatal circumstances (e.g., shared intrauterine environment, premature delivery) render twin study findings inapplicable to non-twin populations. Christensen, Vaupel, Holm, and Yashin (1995) reported that after 6 years of age, disease incidence and mortality are comparable for twins and singletons, a finding confirmed for most other behavioral and physical measures. Twin studies’ reputation has additionally suffered from serious misuse of the methodology by some individuals.

Josef Mengele’s horrific medical experiments using twins, dwarfs, and individuals with genetic defects, conducted in the Auschwitz concentration camp between 1943 and 1945, are exemplary (see Segal, 1985a, 2005a). Viola Bernard’s intentional separation of adopted infant twins and Dr. Peter Neubauer’s (Neubauer & Neubauer, 1990) longitudinal study that took unfair advantage of these twins and their families also hurt the ability of other researchers to make constructive use of twin research (see Perlman, 2005; Segal, 2005b). Another controversy concerned the truthfulness of the reared apart twin IQ data gathered by Cyril Burt, although that situation was ultimately resolved in his favor (Fletcher, 1991; Joynson, 1989).

Scientific sources no longer cite Burt’s studies, but since his results were consistent with current findings their omission does not affect interpretations or conclusions concerning influences on general intelligence. Twin research has survived these challenges as evidenced by increased applications of this approach across many behavioral, medical, and social science fields. Its recovery was due, in part, to advances in genetic research and growing disillusionment with environmental explanations of human behavior and development (Vandenberg, 1969). The molecular structure of DNA was identified in 1953, enhancing understanding of the transmission and expression of genetic factors. The genetic underpinning of Down’s syndrome (trisomy 21) and the metabolic mechanism associated with the mental retardation caused by phenylketonuria (PKU) drew attention to gene-behavior relationships. Social explanations of abnormal behavior became less satisfying, thus renewing attention to biological components of mental disorder. This altered research climate was conducive to some landmark studies of general intelligence.

Erlenmeyer-Kimling and Jarvik (1963) surveyed the twin and adoption literature and concluded that genes substantially influence mental ability. Since then, more extensive updated analyses have been completed by Bouchard and McGue (1981, 1993), with the same results. In 1988, Snyderman and Rothman showed that the majority of behavioral science researchers endorsed genetic influence on intelligence. Today, some researchers are searching for links between specific genes and mental ability and disability, and some promising leads have been found.

For example, Haarla, Butcher, Meaburn, Sham, Craig, and Plomin (2005) found associations (albeit, modest) between DNA markers and general cognitive ability in 7-year-old children. Behavioral genetics, of which twin research is a critical component, entered the mainstream of psychological research in the 1980s and has stayed there. Much of what is currently known about the bases and progression of general intelligence, special mental abilities, Alzheimer’s disease, and associations between earnings and education comes from twin-based analyses. The most important recent advances in twin research include elaboration of twin research designs, greater availability of population-based twin registries, increased sophistication of analytic methods, and new insights on epigenetic processes. Research Designs and Findings: Using Twins to Find Genetic and Environmental Influences on General Intelligence Twin-Singleton Differences The question of possible intellectual differences between twins and singletons is important. That is because it only makes sense to think of twin-based estimates of genetic and environmental effects as applicable to the general population if twins can be considered typical of that population for the trait in question.

The older psychological literature. Includes a number of studies showing a five- to ten-point IQ disadvantage for twins, relative to non-twins (Bouchard & Segal, 1985; Segal, 2000a).

Explanations for this difference refer mostly to twins’ lower average birth weight (due to premature delivery) and close social relationship that could restrict their range of learning opportunities. However, twin studies have revealed only modest birth weight-IQ correlations.

Interestingly, MZ twins show greater birth weight differences than DZ twins, but this pattern tends to reverse itself by 3 months of age (Wilson, 1986). Wilson (1979) also found that co-twins in ten MZ pairs differing in birth weight by over one and three-quarter pounds did not show pronounced IQ differences at 6 years of age, although they did maintain their size difference. He suggested that “a high degree of buffering for the nervous system against the effects of malnutrition” in viable fetuses may protect against early insult (p. It has also been found that fetuses with modest nutritional deficits may show accelerated development of their lungs and brain (Amiel-Tison & Gluck, 1995), possibly explaining the low birth weight-IQ correlations from twin studies.

Low birth weight in singletons has been linked to later cognitive difficulties (Caravale, Tozzi, Albino, & Vicari, 2005; Davis, Burns, Wilkerson, & Steichen, 2005), but low birth weight in twins may not predict comparable developmental delays in otherwise healthy twins. It is possible that the birth weight difference as a percentage of total weight may be a more meaningful factor in individual pairs.

A number of recent studies have explored associations between birth weight and intelligence in twins. Boomsma, van Beijsterveldt, Rietveld, Bartels, and van Baal (2001) found that genetic factors mediate the link between birth weight and IQ in twins until 10 years of age. An association between intrapair differences in birth weight and IQ was detected for DZ twins (who differ genetically), but not for MZ twins.

Luciano, Wright, and Martin (2004), in a study of 16-year-old twins, found that genetic variance in birth weight overlapped with genetic variance in verbal IQ, but not with non-verbal or overall IQ. It was suggested that verbal IQ may serve as a proxy for parents’ education or intelligence and that higher-IQ mothers may provide more favorable intrauterine environments for their children than lower-IQ mothers.

It is important to note that most studies comparing the intellectual levels of twins and singletons have focused on the IQ scores of young children, comparing them to those of unrelated non-twins. This approach fails to control for biological and experiential family background measures.

The more recent literature presents a more varied picture of twins’ intellectual abilities. Posthuma, De Geus, Bleichrodt, and Boomsma (2000) compared the adult IQ scores of MZ and DZ twins with those of their singleton siblings, circumventing some problematic features of earlier studies. No IQ difference between the two groups was found in this study, suggesting that twin studies provide informative estimates of IQ heritability. The twins were part of the Netherlands National Twin Registry and were recruited as adults through City Councils and notices in newsletters. Twins in the study had taken an IQ test as part of a previous study of adult brain function, minimizing effects of self-selection. Findings contradicting those of Posthuma et al.

Were recently reported by Scottish investigators. Ronalds, De Stavola, and Leon (2005) also compared IQ scores for twins and their non-twin siblings, born between 1950 and 1956; thus, twins in this sample were younger than those used by the Dutch investigators. In this study twins scored 5.3 and 6.0 points below their singleton siblings at ages 7 and 9, respectively. Adjusting for sex, mother’s age, and number of older siblings did not affect the data; however, adjusting for birth weight and gestational age reduced the IQ differences to 2.6 and 4.1 points at 7 and 9 of age, respectively.

(See also Deary, Pattie, Wilson, & Whalley, 2005.) A problematic feature of the Scottish study was failure to differentiate MZ and DZ twins. Given that MZ twins are more likely to be exposed to adverse prenatal factors than DZ twins, it is possible that combining MZ and DZ pairs actually decreased twin-singleton differences. It is also possible that better medical care than was available to twins born in the 1950s would reduce, or eliminate, twin-singleton differences among more recently born twins.

However, this cannot be the full explanation because MZ and DZ twins in the Dutch study (who did not differ from their siblings in IQ) were 39.7 and 37.3 years of age, respectively. As such, some twins were born in the 1960s when medical technology was less effective than it is today. Consistent with the Dutch findings are those from a more recent Danish study by Christensen, Petersen, Herskind, and Bingley (2006) that did not detect twin-singleton differences in general intelligence, using school children from a nation-wide population register. The varied results from the recent twin-singleton studies call for additional twin-singleton comparisons of IQ, using representative samples of twin children and adults. In the area of language development, however, twins’ average deficits relative to non-twins have been well documented (Segal, 2000a).

A number of young twins display language delays that are explained mostly by postnatal family influences (e.g., patterns of parent–child communication and interaction), rather than by birth and delivery factors (Rutte, Thorpe, Greenwood, Northstone, & Golding, 2003; Thorpe, Rutter, & Greenwood, 2003; also see Segal, 2000a). Research shows that parents of twins direct less speech to each child and are more controlling in their verbal interactions, relative to mothers of non-twins (Tomasello, Mannle, & Kruger, 1986). Thorpe et al.

(2001) have described two language features: private language (communication used exclusively within pairs, but which is unintelligible to others) and shared verbal understanding (communication used both within pairs and with others, but which is unintelligible to others). Shared verbal understanding was observed among 50 and 19.7% of twins at 20 and 36 months of age, respectively, and among and 2.5 and 1.3% of non-twins. Private language was observed among 11.8 and 6.6% of twins at 20 and 36 months of age, respectively, and among 2.5 and 1.3% of non-twin pairs.

Children showing these speech characteristics scored lower on most cognitive ability measures than those who did not, especially children showing private language at age 36 months. Shared verbal understanding is, however, considered a not uncommon developmental feature in twins and in near-in-age siblings. Children showing such language delays usually recover by age 3 years. However, language delays could partially explain the lower average IQ scores observed among some young twin samples.

The recent dramatic increase in the twinning rate should facilitate additional cognitive comparisons between twins and non-twins. Twins currently occur in approximately 1 in 30 births, as compared with 1 in 50 to 1 in 60 births in 1980 (Center for Disease Control, 2003). Reasons for this increase are important with respect to analyses of general intelligence in twins and non-twins. The rise in twinning is due mostly to the greater availability of artificial reproductive technologies (ART) that enable infertile couples to conceive, although delayed childbearing (associated with DZ twinning) explains part of the trend. Most twins conceived via ART are DZ, although a minority of MZ twins is thought to result from splitting of the embryo due to its micromanipulation outside the womb (Hecht & Magoon, 1998). Studies of artificially conceived non-twin infants have indicated early delivery, low birth weight, and developmental delays (Stromberg et al., 2002).

This raises the possibility that using ART twins in behavioral genetic studies might bias estimates of genetic and environmental influence on measured traits. Studies have yielded mixed findings in this regard. Some investigators have reported no differences in birth and health outcomes between naturally and artificially conceived twins (Helmerhorst, Perquin, Donker, & Keirse, 2004; Tully, Moffit, & Caspi, 2003).

However, other studies have found an increased risk of birth defects (Kuwata et al., 2004) and lower birth weights and reduced co-twin resemblance in birth weight and problem behaviors among artificially conceived twins, relative to naturally conceived twins (Goody et al., 2005). Continued comparison of these twin groups will be important in studies exploring biological and experiential factors affecting mental ability. Clearly, relationships among prenatal factors, health status, and complex behaviors such as general intelligence are not straightforward (Segal, in press, 2009). Twins and singletons experience different biological and social situations affecting their development. Recall, however, that once children reach the age of 6 years there do not appear to be meaningful differences between twins and singletons that would prevent generalizability of twin studies findings to non-twins (Christensen et al., 1995).

Twin-Family Designs Families constructed from MZ twins, their spouses, and children yield a range of genetically and environmentally informative relationships. Twin aunts and uncles are genetically equivalent to genetic mothers and fathers, and first cousins are genetically equivalent to half-siblings. This design has been used to study a variety of traits, such as birth weight (Magnus, 1984), non-verbal intelligence (Rose, Harris, Christian, & Nance, 1979), schizophrenia predisposition (Gottesman & Bertelsen, 1989) conduct disorder (Haber, Jacob, & Heath, 2005), and social closeness (Segal, Seghers, Marelich, Mechanic, & Castillo, 2007). Rose (1979) found evidence of genetic effects on non-verbal ability, given the higher parent–child and twin parent–niece/nephew correlations, relative to spouse uncle/aunt–niece/nephew, and spouse–spouse correlations. Maternal effects were not present. Twins Reared Apart and Together Studies of the rare sets of MZ twins reared apart (MZA) yield direct estimates of genetic influence on measured traits.

If co-twins experience little or no social contact until reunion in adulthood and are raised in uncorrelated environments, their similarity is associated with their shared genes. Bouchard (2005) has asserted that “With monozygotic twins reared apart, correlation is, in fact, an estimate of causation, and the magnitude of the correlation tells you the effect of the genes” (p. DZ twins reared apart (DZA) constitute an important control group, providing opportunities to assess interactions between genotypes and behavior (Segal, 2005c). Unfortunately DZA pairs were not included in studies prior to the 1970s. Separated twin studies have been conducted, or are underway, in the United States (Bouchard, Lykken, McGue, Segal, & Tellegen, 1990; Newman, Freeman, and Holzinger, 1937), Great Britain (Shields, 1962), Denmark (Juel-Nielsen, 1965), Japan (Hayakawa, Shimizu, Kato, Onoi, & Kobayashi, 2002), Sweden (Pedersen, McClearn, Plomin, & Nesselroade, 1992), and Finland (Kervinen, Kaprio, Koskenvuo, Juntunen, & Kesaniemi, 1998; also see Segal, 2003). Most studies are comprehensive, including a wide array of behavioral and physical measures (Segal, 2000a); however, the present discussion focuses on analyses of general intelligence. A remarkable level of consistency has been demonstrated in the magnitude of the intraclass correlations for general intelligence reported across studies, with MZA correlations for primary tests ranging from 0.68 to 0.78 (Bouchard et al., 1990).

This is especially impressive given that reared apart twin studies span multiple age groups, protocols, countries, and cultures. Organizing the studies according to time of publication yields mean IQ correlations of 0.72 (“old data,” n = 65; three small reared apart twin studies conducted between 1937 and 1966) and 0.78 (“new data, n = 93; two relatively larger reared apart twin studies conducted in 1990 in Minnesota, and in 1992 in Sweden); see Plomin, DeFries, McClearn, and McGuffin (2001). These findings indicate that heritable factors explain 72–78% of the variance in general intelligence. This value exceeds the 50% heritability based upon young twin, sibling, and parent–child pairs.

This may be due to the fact that IQ heritability increases with age (discussed below), and MZA twins are studied mostly as adults. MZA twin similarity in any trait is best viewed against the extant data for other genetically and environmentally informative kinships. These data, displayed in Fig. 6.1, show a trend toward increasing IQ similarity with increasing genetic relatedness. Several features in the graph deserve attention. First, MZ twins reared together (MZT) show slightly greater resemblance (0.86) than MZAs (0.78).

One explanation is that growing up together may enhance IQ similarity between MZ co-twins, albeit slightly. However, recall that most studies of reared together twins include young pairs living at home, when the shared environment exerts its greatest effect on development. In contrast, MZA data are typically gathered when twins are reunited as adults. Note that DZ twins reared together (DZT) also show slightly greater resemblance (0.60) than DZA twin pairs (0.52; see Pedersen, McClearn, Plomin, & Friberg, 1985), possibly for the same reasons.

Second, both DZA and DZT pairs show greater resemblance than ordinary full siblings (0.47) even though all pairs share half their genes, on average, by descent. This could conceivably reflect shared age (DZAs and DZTs) and/or shared environmental factors (DZTs). A third intriguing effect shown in Fig. 6.1 is the reduced IQ correlation for full siblings reared apart (0.24).

It is impossible to rule out differences in rearing as explanatory. Another possibility is that the reared apart siblings include a number of half-siblings, due to multiple paternities. Explaining MZA twin similarity in intelligence has taken several routes. Taylor (1980) asserted that four classes of environmental similarities (age at separation, age at reunion, rearing by relatives and social environments) explained IQ resemblance among reunited twin pairs in the three earliest studies. Subsequent to Taylor’s publication, Bouchard (1983) failed to constructively replicate his findings using the alternate IQ measure in each study. (The investigators had obtained more than one measure of general mental ability.) More recently, examining characteristics of MZA twins’ rearing families has failed to yield meaningful associations that would challenge genetic interpretations of mental ability. Specifically, Bouchard et al.

(1990) and Johnson et al. (2006) found negligible correlations between twins’ IQ similarity and similarity in childhood physical facilities in the home, social status indicators, and parenting practices. Frequency of contact between MZ twins prior to assessment was also unrelated to their IQ similarity. It would, however, be incorrect to claim that MZA twin studies eliminate a role for experiential influences on intellectual development. Newman, Freeman, and Holzinger (1937) reported correlations of 0.79 and 0.55 between co-twin differences in educational measures and Binet IQ and Otis IQ scores, respectively, and correlations of 0.51 and 0.53 between co-twin differences in social environments and IQ scores. Note that these are within-pair measures so they indicate that educational and social factors can affect individuals’ intellectual development. At the same time, the reared apart twins’ IQ correlations were 0.67 (Binet IQ) and 0.73 (Otis IQ).

These are between-pair measures so they demonstrate genetic effects. It would seem impossible to explain the intellectual similarities between MZA twins, relative to unrelated siblings reared together, without reference to genetic factors. It is likely that twins’ genetically based predispositions partly explain their tendencies to seek similar opportunities and experiences in their separate environments, illustrative of active gene–environment correlation. It is also likely that parents and significant caretakers respond to individual twins’ preferences and abilities by providing them with meaningful and relevant opportunities, a process termed reactive gene–environment correlation. Unfortunately, the clearest picture of such processes would be available from prospective longitudinal studies of reared apart twins, research that is not practically and ethically feasible. IQ findings summarized in Fig. 6.1 also include data on a novel, relatively unstudied kinship called virtual twins, describedbelow.

Virtual Twins Virtual twins (VT) are same-age unrelated children, reared together since infancy (Segal, 1997, 2000a; Segal & Hershberger, 2005). They fall into two classes: adopted–adopted pairs and adopted–biological pairs. These unique sibships mimic the situations of MZ and DZ twins, but without the genetic link; of course, VTs are somewhat more akin to DZ twins who do not look physically alike. Another way to think about VTs is that they represent the reverse of MZ twins reared apart because the former share environments, not genes, while the latter share genes, not environments. VTs offer researchers a valuable “twin-like” design for studying the extent to which shared environments influence general intelligence and other phenotypes (Segal, 2000a, 2000b; Segal & Allison, 2002). Specifically, they circumvent some problematic features of ordinary adoptive siblings who differ in age, time of entry into the family, and often in placement history. It is possible, for example, that resources in the home might differ for two children adopted several years apart, benefiting one child but not the other.

A recent report of IQ resemblance included 113 VT pairs, with a mean age of 8.10 years (SD = 8.56) and age range of 5–54 years. (About 70% of the pairs were younger than 7 years of age. The remaining 30% included twin children and adolescents, and seven pairs aged 22 years and older.) The mean age difference between the pairs was 3.10 months (SD = 2.80) and ranged between 0 and 9.20 months.

Intraclass correlations were 0.26 for full-scale IQ score, 0.23 for verbal IQ score, and 0.21 for performance IQ score. The profile correlation across IQ subtests was 0.07.

Thus, shared family environment is associated with modest intellectual similarity among family members during childhood. These results are best appraised against the backdrop of findings for MZ and DZ twin pairs. IQ correlations for MZ (n = 4, 672) and DZ twin pairs (n = 5, 533), averaged across a number of studies, were 0.86 and 0.60, respectively (see Fig. A study of 7- to 13-year-old twins (comparable in age to the VTS) reported IQ correlations of 0.85 for MZ twin pairs (n = 69) and 0.45 for DZ twin pairs (n = 35 pairs), and profile correlations (across subtests) of 0.45 for MZ twin pairs and 0.24 for DZ twin pairs (Segal, 1985b). It is noteworthy that the VT IQ correlation (0.26) is nearly identical to the weighted average adopted sibling correlation of 0.25 (McGue, Bouchard, Iacono, & Lykken, 1993). It could be argued that efforts should be directed toward studying ordinary adoptive siblings, rather than the rare VT pairs.

However, given the controversies still surrounding genetic explanations of intelligence, it is important to control for environmental features that could potentially affect ability to gather the most environmentally informative pairs possible and to identify novel kinships for replication of findings. Additional IQ analyses are possible using VTs because of the availability of both biological and adoptive children. Biological children of the parents of virtual twin pairs scored significantly higher in full-scale IQ, verbal IQ, and performance IQ, relative to the adoptive children. This finding is consistent with other adoption studies (see, for example, Cardon, 1994; Dumaret & Stewart, 1985), although the bases for this difference are uncertain. The majority of parents in the VT study held professional or managerial positions, so it is possible that their biological children inherited predispositions for advanced intellectual skills.

In contrast, the adoptive children may have come from more varied biological family backgrounds. Of course, relationships between adoption and later behavioral development are complex, as shown by a recent meta-analysis by Van Ijzendoorn, Juffer, and Poelhuis (2005). It was found that adopted children’s IQ scores exceeded those of their non-adopted (biological) siblings and their peers raised by the birth family or placed in institutional care. Their school performance was also better. Second, adopted children’s IQ scores did not differ from those of their non-adopted siblings (the children with whom they were raised) or those of their current peers; however, they performed less well at school, showed poorer language skills and required more special education referrals. As the investigators concluded, adoption has positive effects on intelligence, but the varied effects of early deprivation, emotional correlates of adoption, and other factors may affect intellectual performance and progress.

IQ assessments continue to be conducted for new VT pairs. In addition, pairs who have already participated are being reassessed. This longitudinal component to the project is part of the TAPS (Twins, Adoptees, Peers, and Siblings) Study, a collaboration between investigators at California State University Fullerton and the University of San Francisco.

A new analysis of the IQ similarity of 43 young VT pairs, retested 1.70–8.96 years after their initial assessment, is now available (Segal, McGuire, Havlena, Gill, & Hershberger, 2007). A decrease in IQ resemblance was observed, suggesting increased genetic and/or non-shared environmental effects and decreased shared environmental effects on general intelligence during childhood. It will be especially interesting to examine VT similarity as siblings approach adolescence, given that previous adoption studies have reported correlations of 0.30 for adopted siblings at age 8, but near zero correlations for adopted siblings in the teenage years (Loehlin, Horn, & Willerman, 1989). Further longitudinal analyses of VTs may shed light on gene–environment interaction effects on IQ if the IQ scores of adoptive children in adoptive–biological pairs begin to approach those of their non-adopted siblings. Longitudinal and Life Span Twin Studies Behavioral geneticists interested in developmental issues segued into developmental behavioral geneticists (although some developmental psychologists and others still resist genetic perspectives on behavior; see Pinker, 2002). Questions of the extent to which genes and environments accounted for continuity and change in intelligence, personality, and physical features were addressed via longitudinal twin studies.

Combining these studies with data on the twins’ singleton siblings added additional informative features. Probably the most widely cited longitudinal twin analysis of intellectual development is Wilson’s (1983) tracking of MZ and DZ twins’ intellectual progress from 3 months to 15 years of age.

Little difference in the magnitudes of the MZ and DZ correlations was apparent at 6 months of age, after which MZ twin pairs showed correlations about 0.10 higher than those of DZ twins during the period between 9 and 36 months. MZ correlations increased steadily from 0.67 (9 months) to 0.88 (36 months), while DZ correlations increased from 0.51 (9 months) to 0.73 (24 months), decreased to 0.65 (30 months), and increased again to 0.79 (36 months). Then, the MZ twin correlations remained stable, with a mean correlation of 0.82 for ages 8–15 years. In contrast, the DZ twin correlations declined except for a slight rise at the age 6 years, yielding a mean correlation of 0.50 from 8 to 15 years of age.

Thus, heritability increased from early childhood (0.16) to adolescence (0.64). Another remarkable outcome from Wilson’s (1983) study was that the twin correlations exceeded the age-to-age continuity, meaning that twin A’s score at a particular age better predicted twin B’s score than a previous score of twin B. Furthermore, plots of individual MZ and DZ pairs depicted coordinated and discrepant patterns of “spurts” and “lags,” respectively, demonstrating genetic influence on intellectual growth patterns. Finally, the twin-sibling correlations confirmed the findings for the DZ twins, i.e., sibling correlations increased from 0.38 at age 3 years to 0.55 at age 7 years, then declined slightly to 0.50 at age 15 years. A number of longitudinal twin studies have followed Wilson’s classic work, so only selected examples will be presented. Spinath, Ronald, Harlaar, Price, and Plomin (2003) showed that genetic influence increased from early childhood (20–30%) to middle childhood (40%), and again with the approach of adolescence (50%).

It was also shown that shared environmental influence on general intelligence declined to near zero across this portion of the life span. These findings have been generally supported by other longitudinal data from twins, as well as from biological siblings and adoptees, gathered from 1 to 12 years of age by Bishop et al.

Two exceptional findings from that study were that (1) non-shared environmental factors were associated with both IQ stability and change in middle childhood and (2) genetic factors were only associated with IQ stability at adolescence. However, Dutch investigators Rietveld, Dolan, van Baal, and Boomsma (2003) found that IQ stability across ages 5, 7, and 10 years was mostly explained by genetic factors and that non-shared environment contributed only to variance that was age specific. Genetically influenced transition times in intellectual development have also been identified via longitudinal twin studies. Fulker, Cherny, and Cardon (1993) reported developmental changes during childhood. Specifically, genetic influence on cognition seemed to stabilize by age 4, with new variation appearing at age 7. Some studies have restricted IQ analyses to twins at the older end of the life span.

These studies are striking in that they also show increasing IQ heritability across the life span. A study of 80-year-old Swedish twins (McClearn et al., 1997) yielded an IQ heritability of 0.60.

Evidence of increasing IQ heritability also comes from cross-sectional analyses, in which the MZ–DZ difference in correlations widens after adolescence, into adulthood (McGue et al., 1993). As in Wilson’s (1983) study, the increasing MZ–DZ similarity difference is explained by the growing discordance between DZ co-twins, relative to the stable concordance between MZ co-twins. Reasons for increasing genetic influence on intelligence across the life span have been considered.

It seems likely that two sources of influence are operative. First, small genetic effects present in childhood may become more important over time, leading to larger behavioral effects. Second, shared environmental influence declines and genetic influence increases as individuals become more active in seeking opportunities for learning and for self-expression (Plomin et al., 2001), especially with the end of required schooling. This would be an example of gene–environment correlation—the concept that certain genotypes are selectively found in certain environments. There is, however, some evidence that heritability declines at the very oldest ages.

An IQ study of Swedish twins aged 50 years and older indicated a heritability of 0.80 (Pedersen et al., 1992), a figure that fell to 0.60 for twins at age 80. This suggests that environmental factors, including physical health, may gain some importance toward the end of the life span. With this in mind, a recent study examined sources of influence on rate of cognitive change, using two groups of older twins (65 years and younger; older than 65 years) from the Swedish study (Reynolds, Finkel, Gatz, & Pedersen, 2002).

Genetic influences were associated with individual differences in ability, whereas environmental factors were more closely tied to rate of change. Trends toward increased longevity will facilitate further efforts along these lines. Environmental Influences on General Intelligence Two main classes of environmental influence on general intelligence are of interest with respect to twin studies: prenatal environments and parenting practices. This is because such effects could bias estimates of heritability if they have a meaningful impact on twins’ intellectual development.

Other potential sources of environmental influence on intelligence (e.g., quality and years of schooling; parental socioeconomic status) have been addressed in the psychological literature and are beyond the scope of the present chapter. However, some comments along these lines will be included at the end of this section.

The puzzle that emerges from the foregoing is that greater resemblance between two-chorion twins, not one-chorion twins, would be expected. This is because one-chorion twins are generally at greater physical risk, due to complications from shared fetal circulation. It is possible that one-chorion twins who survive and volunteer for research represent a remarkably healthy subgroup of such sets (Segal, 2000a), but this remains speculative. Perhaps there are, as yet, unidentified features associated with delayed zygotic splitting conducive to co-twins’ matched phenotypic development in some domains. One candidate with respect to female twins would be X-inactivation patterns, for which late-splitting twins show greater concordance; this would lead to greater co-twin resemblance in X-linked traits (Trejo et al., 1994). The important point is that placentation should be considered in twin-based analyses of human behavior. Further efforts along these lines may shed light on the mechanisms underlying the differential resemblance between MZ twin types, thus refining estimates of heritability.

Parenting and Twin Studies: Effects on Intelligence Examining parenting practices can help refine heritability estimates by revealing whether treatment of twins is causal or reactive in nature. As indicated above, the view that similar treatment of MZ twins produces similar behavioral outcomes has caused some individuals to question results from twin research. Of course, the key question is not whether MZ twins receive more similar treatment than DZ twins, but whether any more similar treatment they receive enhances their phenotypic resemblance (Rowe, 1994). A substantial number of studies have variously assessed the similarity of parenting effects, physical similarity, and childhood experiences on twins’ similarity in personality (Borkenau, Riemann, Angleitner, & Spinath, 2002), eating behaviors (Klump, Holly, Iacono, McGue, & Wilson, 2000), behavioral problems (Cronk et al., 2002; Morris-Yates, Andrews, Howie, & Henderson, 1990), and psychiatric illness (Kendler, Neale, Kessler, Heath, & Eaves, 1994).

These studies have found that environmental similarity was largely unrelated to twins’ similarity in the measured traits, thus affirming the equal environments assumption. Fewer studies have assessed the effects of rearing on intelligence, but those that have done so have drawn the same conclusion.

In their landmark study of 850 twin sets, Loehlin, & Nichols (1976) found negligible correlations between twin differences in NMSQT measures and differential experience measures. The same pattern held for interests. Recall that the twins reared apart data (reviewed above) also indicated little effect of family background variables on twins’ IQ scores. Another informative series of analyses has compared parents’ judgments of twins’ behaviors when parents were correct and incorrect about twin type. The majority of such studies have found parental ratings to be consistent with true twin type, rather than with assumed twin type (Scarr, 1969; Goodman and Stevenson, 1991). Goodman and Stevenson (1991) did, however, find that the majority of parental warmth and criticism was unrelated to the child’s behavior.

Such studies have not been conducted with reference to cognitive skills, but there is little reason to suspect that they would deviate from the patterns found for behaviors in other domains. In concluding this section, a study by Turkheimer, Haley, Waldron, D’Onofrio, and Gottesman (2003) is worth noting. It was found that heritability estimates of general intelligence were close to zero among 7-year-old twin children from impoverished families and that 60% of the IQ vari-twins with a shared chorion and placenta, but separate amnions. D: MZ twins with a shared chorion, amnion and placenta. Adapted from Potter (1948) and Stern (1960); see Segal (2000a). Ance was associated with shared environmental factors. In contrast, the reverse was true for twin children from affluent families.

As the authors indicated, it would be inappropriate to claim that behavioral differences among children from poor environments are more closely tied to their environments than are outcome differences among children from favorable environments. This is because the genetic influences that varied with socioeconomic status were only those that were independent of socioeconomic status. It is possible, if not likely, that a substantial portion of the genetic influences on general intelligence are common to genetic influences on socioeconomic status (SES). If so, these genetic influences were not measured in this study at all; also see Plomin et al. Such common genetic influences as those referenced above would occur, for example, if parents who attain high levels of education and income do so because they have high intelligence and, therefore, pass these genes to their children along with their high SES environment.

Samples of reared-apart twins and/or samples allowing for the measurement of intergenerational transmission and the separate measurement of SES, or its effects in co-twins, will be necessary to estimate all genetic and environmental associations involved. This is an important direction for future research. Endophenotypes: What Can They Tell Us?

General mental ability is typically assessed by evaluating composite performance on a number of mental tasks requiring diverse knowledge, skills, and reasoning (Jensen, 1998). General mental ability is, therefore, a highly abstract concept. It reflects not only the complex behaviors involved in solving cognitive ability problems that we can see, but also the variance common to a variety of these behaviors. Thus, the gap between the gene products and the environments in which they are formed, and the “hidden” genetic and environmental influences on general mental ability we measure through most twin and related studies is large.

In order to understand how genes and environments transact to create the mental ability performances we observe as behavior, we need to understand the biological processes lying within this gap. The concept of the endophenotype is potentially useful in this regard. Twin studies can contribute importantly to identifying endophenotypes and investigating their roles in the development of general mental ability. The term endophenotype was adapted by Gottesman and Shields (1973) from evolutionary theory involving insect biology to describe the “internal phenotypes discoverable by a biochemical test or microscopic examination” involved in schizophrenia (Gottesman and Gould, 2003, p. The term refers to biological mechanisms thought to be closer to the immediate products of genes and, thus, under stronger and perhaps less polygenic genetic influence than are the manifest behaviors they undergird. For example, the inability to synthesize phenylalanine would be considered an endophenotype for PKU (phenylketonuria)-induced mental retardation.

The idea that there are neurological and biochemical bases of general mental ability and other psychological features did not originate with Gottesman. However, the term endophenotype allows for clearer characterization of some of the roles played by neurological factors in psychological manifestations. Gottesman intended the concept of endophenotypes to be specific to genetic influences resulting from DNA sequence variations. He did this because the specific purpose of identifying endophenotypes is to assist in the search for particular genes involved in a behavior. Once accomplished, this can further understanding of how those genes transact with the environment to result in the biological processes involved in the phenotype. Gottesman and Gould (2003) specified five criteria for the designation of endophenotypes. These criteria were, however, developed to apply to schizophrenia, a behavioral pattern that can be considered an overt disorder and for which irregularities in biological processes have been identified as endophenotypes.

In contrast, general cognitive ability is a clearly overtly continuous trait for which the concept of a threshold of disorder is less clearly applicable. Endophenotypes of cognitive ability are also more likely to be continuous. This renders two of Gottesman and Gould’s criteria irrelevant. The remaining relevant criteria are (1) the endophenotype is associated with general mental ability in the population; (2) the endophenotype is heritable; and (3) the endophenotype and general mental ability are related within families, as well as throughout the population. 1 This is equivalent to saying that there are common genetic influences on the endophenotype and on general mental ability. 1 The other two criteria were that the endophenotype is present regardless of whether or not the disorder is currently present, and the endophenotype is present at a higher rate in the unaffected relatives of people with the disorder than in the general population.

Identification of possible endophenotypes related to general mental ability has proceeded along the lines earmarked by the first two criteria. First, studies have sought to relate brain structure and function to general mental ability. Twin studies are of limited value here, but we review this area because of its obvious importance to the overall process of identifying endophenotypes for general mental ability. Early studies in this area were based on patients with brain damage, and patient–control studies still provide important data.

However, more recent studies have focused on assessments of normally functioning groups of participants. These studies have been sufficiently successful such that studies of brain structures and malfunctions involved in disease now routinely correct for estimated mental ability, prior to inception of the disease state (Gray & Thompson, 2004).

Magnetic resonance imaging (MRI) studies have revealed structural associations. There are substantial correlations between general mental ability and total brain volume and total volumes of both gray and white brain matters, as well as volumes of gray and white matter in specific brain areas, particularly those in the frontal and parietal lobes involved in language (Haier, Jung, Yeo, Head, & Alkire, 2004; McDaniel, 2005). More provocatively, one study (Pennington et al., 2000) found that the volumes of 13 brain regions were substantially intercorrelated, with a general factor accounting for 48% of the variance. This suggests a general structural factor linked to the general mental ability factor. In addition, functional studies of neural activity using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have revealed areas of the brain involved in intelligent performance. For example, Duncan et al.

(2000) found that three tasks (Spatial, Verbal, and Circles) requiring different kinds of mental ability were associated with greater neural activity in several brain regions, but only one area (the lateral prefrontal cortex) was activated during all tasks. This finding of a central location for general mental ability, however, contradicts the findings of several fMRI studies (Esposito, Kirby, Van Horn, Ellmore, & Berman, 1999; Gray, Chabris, & Braver, 2003; Prabhakaran, Smith, Desmond, Glover, & Gabrielli, 1997) that have reported widespread activity throughout the brain during cognitive tasks. The contradiction may reflect the different technologies used and may be more apparent than real with respect to hypotheses regarding unitary general versus multiple intelligences.

This is because the functional units of higher cognition may include networks of brain areas as well as single areas (Gray & Thompson, 2004). This discussion of work to date shows that we have identified total brain size, regional brain size, and level of brain activity as potential endophenotypes of general mental ability.

Of course, the key to identifying these features as actual endophenotypes is that they share common genetic influences with general mental ability. This has been investigated in only one study. Importantly, however, it was a twin study, and this is another area in which twin studies will prove to be important in future research.

Posthuma et al. (2002) found that the observed correlations between general mental ability and volumes of gray and white brain matter in a sample of Dutch twins and their siblings were due completely to genetic influences. Thus, volumes of gray and white matter can be considered endophenotypes for general cognitive ability. This means that, as we identify genes controlling the development of gray and white brain matter and come to understand the processes involved in their expression, we should build directly on our understanding of the biological development of general mental ability, as well. Given that the correlations between brain matter volumes and general mental ability are on the order of 0.3, however, we should expect that we will need to identify other endophenotypes and perhaps environmentally based biological processes, as well, before we fully understand the development and manifestation of general mental ability. Genes for General Mental Ability: Can We Find Them? The fact that general mental ability is heritable means that its variation in the population arises, at least in part, from variations in DNA.

These variations, called polymorphisms, are passed from parents to their offspring. However, two randomly selected individuals will show differences in only 0.1–0.2% of the nucleotides in their genome (Thompson et al., 2001).

Many of these differences likely have little relevance to general mental ability. In addition, relatively few actually occur in protein coding regions. This emphasizes an important point in the search for specific genes involved in general mental ability: the brain is a complex organ whose function is absolutely essential to the organism throughout its life.

Together, this complexity and vital importance imply the existence of considerable redundancy of genetic control of biological function. This is because they suggest the existence of multiple genetic mechanisms through which any essential brain functions can be maintained. Evidence for this kind of redundancy of function is routinely provided by the regenerative and compensatory capacities shown by people suffering brain damage due to stroke, injury, or disease (Beatty, 1995). It is also routinely provided by non-human studies involving animals in which particular genes have been removed or rendered inactive. Such animals, nevertheless, effectively function normally despite absence of the inactivated gene. This kind of redundancy clearly complicates the search for specific polymorphisms associated with general mental ability. This is because it suggests that polymorphisms associated with high or low function in one family may not even be present in another family, due to the different possible pathways to high or low function.

At the same time, building a brain is a general process, as the actual and potential endophenotypes that have been identified to date make clear. To the extent that quantitative characteristics (e.g., volumes of total, gray, or white brain matter) or specific kinds of brain activity are associated with general mental ability, there are unlikely to be specific polymorphisms that contribute directly to variation in general mental ability. This is because everyone’s brain contains both gray and white matter, and everyone’s brain shows activity during task performance. Thus, genes that contribute directly to the formation of brain matter or the elicitation of activity will be unlikely to segregate among individuals. Instead, phenotypic variations from individual to individual may result from differences in expression by these genes.

These differences, in turn, could be due to differences in other genes that regulate their expression, to environmental influences on gene expression, or both. Much of the work involved in investigating these kinds of genetic processes is carried out in experimental animals in which genetic background can be closely controlled. In humans, twin studies provide the closest scientific and ethical alternatives. For example, evidence for differential environmental effects on genetic expression in rats is provided by Weaver et al. (2004), who described long-term differences in offsprings’ hypothalamic–pituitary–adrenal response to stress.

These differences appeared to result from differences in DNA methylation of a glucocorticoid receptor gene promoter in the hippocampus, brought on by differences in maternal licking, grooming, and nursing practices. The differences appeared to contribute to parental behavior by the offspring as well, leading to differences in stress response that were transmitted from generation to generation. It is unknown whether the particular genetic and environmental mechanisms involved in this process in rats have directly analogous mechanisms in humans.

It is highly likely, however, that this kind of process takes place in humans, contributing to differences in genetic expression that cannot be observed through examination of DNA samples. The fact that genetic expression also appears to be subject to genetic influence (York et al., 2005) complicates this picture further, as it suggests another means by which genetic influences on general mental ability may be rather indirect. It is also possible that some environmental stresses may elicit expression of genetic variation that has lain dormant in the population. This has been demonstrated in Drosophila melanogaster through the generation of phenocopies. Phenocopies are phenotypes that closely match known genetic mutations, usually aberrant. They are, however, generated in genetically wild-type animals by delivering particular stresses during specific developmental periods (Rutherford, 2000). For example, at 21–23 hours of pupal development, 4 hours of heat treatment disrupts the posterior cross-veins in a small percentage of flies (Waddington, 1957).

This deleterious phenotype is very similar to the effects of mutations in the cv gene. Heat stress-induced phenocopies do not result from mutations, however, but rather from the interactions of several to many other genes with the stressful environment. Waddington (1953) demonstrated the heritability of this effect in a series of well-known experiments by crossing the specific animals affected by the heat treatment and subjecting their progeny to it again.

In response to this selection, the proportion of affected animals produced each generation increased until nearly all the animals receiving the treatment showed the effect. At this point, some fraction of the control flies from the same selection lines expressed the effect even without receiving the treatment. Thus, previously silent genetic variation revealed by environmental stresses can be selected to the point at which the stress-induced phenotype is reliably expressed even in the absence of the stress.

Again, it is highly likely that similar processes take place in humans, generating situations in which general mental ability is possibly associated with certain genes in some individuals or groups, but not in others. Some evidence that this might be the case has been provided by Turkheimer et al. (2003), who found, as noted above, that, in young children, genetic variability in IQ independent of SES increased with socioeconomic status. In humans, MZ twins do provide information about another mechanism that complicates the search for genes for multigenic traits such as general mental ability, in an interesting twist on the classical twin study method that focuses on twin similarity. Though MZ twins share a common genetic background, which includes genetic influence on gene expression, significant variation in gene expression remains. The extent of this variation increases with age (Fraga et al., 2005), suggesting environmental influences. It tends to be found in genes involving signaling and communication or immune and related functions (Sharma et al., 2005), implicating the involvement of general mental ability due to the general, brain-wide nature of its known endophenotypes.

Comparing the similarity of MZ twins reared together and apart across multiple traits suggests, however, that post-natal environmental experiences are not the only sources of these epigenetic differences. This is because MZ twins tend to be similar to the same degree in many traits regardless of whether they are reared together or apart (Wong, Gottesman, & Petronis, 2005). This “similarity of similarity” may be due to gene–environment correlation, or the tendency for genetically similar individuals to seek out similar environments and experiences. It may also be due to the tendency for additional resemblance due to shared rearing environments to be offset by social differentiation, or the intentional selection of different experiences, by members of twin pairs growing up together (Segal, in press). In addition, in laboratory animals for which even prenatal environments can be tightly controlled, such epigenetic differences persist in genetically identical organiams, i.e., clones (Gartner, 1990). At the same time, Gartner and Baunack (1981) carried out an interesting experiment using genetically identical cloned mouse pairs, controlling the environment to the same degree. The mouse pairs included MZ and DZ sets, created by transplanting divided and non-divided eight-cell embryos into pseudo-pregnant surrogates.

Specifically, MZ pairs were created by artificial separation of embryos, while DZ pairs were created from multiple zygotes produced by inbred mice. Thus, each pair was genetically identical and reared in the same environment. There was more phenotypic variation within the DZ pairs than within the MZ pairs. The investigators termed this variance the “third component” after genes and environment, but its molecular basis remains unknown. It is clear, however, that there is more to epigenetic effects than simply the accumulation of different experiences over a lifetime.

Still another potential complication arises from the operation of standard kinetics in multi-step steady-state systems, illustrated nicely with an example from human and mouse blood pressure, recounted by Smithies (2005). The renin–angiotensin system is generally acknowledged to be one of the most important means through which blood pressure is genetically controlled. In this system, renin (produced in. The kidney) acts on angiotensinogen (AGT, produced in the liver) to generate angiotensin I. This is converted by the enzyme angiotensin-converting-enzyme ACE to angiotensin II, which acts through several different receptors to increase blood pressure.

Using gene-titration to increase the numbers of copies of the genes for AGT and ACE in mice artificially, Kim et al. (1995) showed that increasing expression of the relevant gene increased the concentration of AGT in the blood and also increased the blood pressure of the mice. This technique also increased the concentration of ACE in the blood, but it had no effect on the blood pressure of the mice (Krege et al., 1997) as ACE effectively acted only as a gatekeeper in the conversion process from angiotensin I to angiotensin II.

The point here is that the effects that variations in genetic expression will have on the outcome phenotype depend on the roles in the underlying biochemical processes, played by each specific gene product. Without complete understanding of these roles we may find it difficult to detect the associated genes. This, of course, is where endophenotypes can be helpful, because they suggest candidate genes. In contrast to this, however, the phenylketonuria (PKU) gene that causes a severe form of mental retardation (unless phenylalanine is removed from the childhood diet) was identified in the 1930s and cloned in 1983 (Woo, Lidsky, Guttler, Chandra, & Robson, 1983), yet we still do not understand how the mutated gene damages brain function. Any consideration of genetic influences on general mental ability has to account for their place in evolution. Two issues are important here. First, general mental ability seems to be part of a very general system of biological processes.

This can be seen by studying monogenic cases of mental retardation caused by deleterious mutations. Some 282 monogenic disorders have been reported to involve mental ability (Inlow & Restifo, 2004). Most, however, were originally identified in relation to their associations with other medical conditions. Furthermore, the ranges of mental ability demonstrated by affected individuals are wide, because they reflect the damaging effects of the mutations, but leave the rest of the normal distribution of ability intact. The genes involve general biological processes such as metabolic and signaling pathways, transcription, and aspects of neuronal and glial biology in highly pleiotropic ways (Inlow & Restifo, 2004). In spite of the fact that such monogenic disorders cause only a small proportion of cases of mental retardation (and only a small proportion of cases of physical illness or disability, as well), they make clear the degree to which general mental ability is integrated into the broader system of biological processes involved in the overall integrity of the organism.

This suggests that the effects of more common genes, though less severe, may be similarly pervasive. As such, the complex mix of genes influencing medical conditions such as hypertension, cardiovascular disease, diabetes, asthma, and multiple sclerosis may influence general mental ability, as well. At the same time, genes influencing over-all health indices, such as immune response and physical growth, may confer benefits for general cognitive ability, as well. Thus, regardless of the direction of their effects, many genes associated with mental ability may be very general in their effects. Those that are positive may have been subject to natural selection, while those that are negative may survive simply because their individual effects are rather small.

Second, and in contrast (though brain function is clearly strongly general), it is obvious that mental abilities can take specific forms such as musical, artistic, or computational talents. Miller (2000) has hypothesized that both general and specific mental abilities have evolved because they have been subject to sexual selection, meaning that they confer advantages to their holders in mate competition. This suggests that some genes involved in mental ability may have very specific, largely ornamental effects that are difficult to identify, because we have yet to develop tests that accurately measure the presence of those abilities. Dragon Ball Z Budokai Tenkaichi 2 Download Ita. In addition, some genes that confer cognitive advantages in the heterozygote may have deleterious consequences in the homozygote. One example of this has been suggested by Cochran, Hardy, and Harpending (2006), who note that several genes involved in DNA repair, including BRCA1 (a gene associated with breast and colon cancer and other autoimmune disorders), have deleterious mutations that have reached polymorphic frequency in Ashkenazi Jews, who also average higher IQ’s than any other human population. They suggest that these mutations have survived in these populations, despite their damaging consequences, because they benefit general mental ability.

This discussion of complications is not intended to discourage the search for specific genes involved in general mental ability. Most studies involving such searches today, however, make use of genetic linkage, association, and scanning techniques, based on the assumption that there is a one-to-one correspondence between genotype and phenotype. Findings from these studies have been difficult to replicate. The complications discussed here may provide some explanations for these failures. At the same time, the existence of these kinds of phenomena underline the richness and intimacy of transactions between genes and environments we are likely to discover as we continue searching for specific genes involved in general mental ability. This reminds us that even when genes associated with general mental ability have been identified, use of genetic modification techniques may have unintended consequences, both positive and negative. Conclusions and Future Directions The preponderance of evidence from classic twin studies, and studies using variant twin designs, is consistent with genetic.

Influence on general intelligence. Given past controversies surrounding this conclusion, it is anticipated that debates over the extent of genetic influence will continue. Capitalizing further on naturally occurring “human experiments,” such as twins reared apart and virtual twins, will be a vital part of future research in this area. It is expected that twins will continue to be used in creative ways in the future, to further address issues and questions concerning the development of general intelligence and its correlates. As an example, Australian investigators used twins to assess the heritability of inspection time (IT) and its covariance with IQ (Luciano et al., 2001). Results yielded a shared genetic factor influencing IT and IQ, separate from the total genetic variance, a finding informative with respect to how individuals process information. Other investigators have looked at co-morbidity between verbal and non-verbal delays in 2-year-old twin children (Purcell et al., 2001).

A major finding was that co-morbidity between the two is largely genetic in origin, whereas differences are largely environmental. It was suggested that such efforts can lead to improved diagnostic systems, based on genetic factors rather than observed symptoms.

Australian researchers found common genetic influence on a standard test of academic achievement and IQ (particularly verbal IQ) in a study of 256 MZ and 326 DZ twin pairs (Wainwright, Wright, Geffen, Luciano, & Martin, 2005). Clearly, twins bring added perspective to research on general intelligence, given the genetic and family background controls. Cross-cultural analyses of intelligence using twins would offer insights into the impact of cultural effects on the heritability of general mental ability. A Russian longitudinal twin study reported a decrease in genetic effects as children transitioned from 6 to 7 years of age (Malykh, Zyrianova, & Kuravsky, 2003). At age 7 shared environmental effects had increased substantially. This line of inquiry would profit substantially by studying the rare group of MZ twins reared separately in different cultures. An ongoing prospective study of young Chinese twins, adopted together and apart, will also shed light on genetic and environmental effects on intellectual development (Segal, Chavarria, & Stohs, 2008).

Identifying genes associated with intelligence may also move ahead as a result of twin studies. Recently, Harlaar et al. (2005) found five DNA markers associated with general cognitive ability in a longitudinal study of 7,414 twin pairs, assessed at ages 2, 3, 4, and 7 tears. Of course, effects sizes of the five markers were quite small. The search for mechanisms to explain how, and why, MZ twins show remarkable similarities in their intellectual development, despite differences in rearing and differences in some brain characteristics, raises intriguing contradictions.

Research on the cerebral development of MZ twins reveals both striking similarities and differences. A study of twins, using magnetic resonance imaging (MRI), showed a 94% heritability of the corpus callosum (CC) midsaggital size (Scamvougeras, Kigar, Jones, Weinberger, & Witelson, 2003). It was suggested that correlates of CC size, such as lateralization patterns, cognitive skills, and neuropsychological functions, could be associated with genetic factors affecting CC morphology.

In contrast, a recent study showed little MZ twin resemblance in the shape of the planum temporale, a brain structure possibly linked to language (Steinmetz, Herzog, Schlaug, Huang, & Jancke, 1995). Furthermore, development of neural structures proceeds according to dynamic processes, some of which may be stochastic, such that MZ twins may conceivably show “functionally significant variant wiring” (Edelman, 1987, p.

Resolution of such disparate findings can enhance understanding of intellectual development in the general population. It is clear that the natural experiment provided by genetically identical twins, who can serve as controls for one another, will continue to provide unique insights into the development and manifestation of mental ability throughout the foreseeable future. W., Cherny, S. S., & Cardon, L.

Continuity and change in cognitive developement. McClearn (Eds.), Nature, nurture, and psychology (pp. Washington, D.

C.:American Psychological Association. The history of twins as a criterion of the relative powers of nature and nurture. Journal of the Anthropological Institute, 5, 391–406. A third component causing random variability beside environment and genotype: A reason for the limited success of a 30 year long effort to standardize laboratory animals?

Laboratory Animals, 24, 71–77. Gartner, K., & Baunack, E.

Is the similarity of monozygotic twins due to genetic factors alone? Nature, 292, 646–647. Goodman, R., & Stevenson, J. Parental criticism and warmth toward unrecognized monozygotic twins. Behavioral and Brain Sciences, 14, 394–395.

Goody, A., Rice, F., Bolvin, J., Harold, G. F., & Thapur, A. Twins born following fertility treatment: Implications for quantitative genetic studies. Twin Research and Human Genetics, 8, 337–345. Gottesman, I. I., & Bertelsen, A.

Confirming unexpressed genotypes for schizophrenia. Archives of General Psychiatry, 46, 867–872. Gottesman, I. I., & Gould, T.

The endophenotype concept in psychiatry: Etymology and strategic intentions. American Journal of Psychiatry, 160, 636–645. Gottesman, I. I., & Shields, J. Genetic theorizing and schizophrenia. British Journal of Psychiatry, 122, 15–30. R., Chabris, C.

F., & Braver, T. Neural mechanisms of general fluid intelligence. Nature Neuroscience, 6, 316–322. R., & Thompson, P.

The neurobiology of intelligence: Science and ethics. Nature Neuroscience, 5, 471–482.

R., Jacob, T., & Heath, A. Paternal alcoholism and offspring conduct disorder: Evidence for the ‘common genes’ hypothesis. Twin Research and Human Genetics, 8, 120–131. A., Head, K., & Alkire, M. Structural brain variation and general intelligence.

NeuroImage, 23, 425–433. J., Siegel, B.

V., MacLachlan, A., Soderling, E., Lottenberg, S., & Buchsbaum, M. Regional glucose metabolic changes after learning a complex visual-spatial motor task: A positron emission tomography study. Brain Research, 570, 134–143. J., White, N. S., & Alkire, M. Individual differences in general intelligence correlate with brain function during nonreasoning tasks.

Intelligence, 31, 429–441. Harlaar, N., Butcher, L. M., Meaburn, E., Sham, P., Craig, I. W., & Plomin, R.

A behavioural genomic analysis of DNA markers associated with general cognitive ability in 7-year-olds. Journal of Child Psychology and Psychiatry, 46, 1097–1107. Hayakawa, K., Shimizu, T., Kato, K., Onoi, M., & Kobayashi, Y. A gerontological cohort study of aged twins: The Osaka University Aged Twin Registry.

Twin Research, 5, 387–388. R., & Magoon, M. Can the epidemic of iatrogenic multiples be conquered?

Clinical Obstetrics and Gynecology, 41, 126–137. Helmerhorst, F. M., Perquin, D. M., Donker, D., & Keirse, M. Perinatal outcome of singletons and twins after assisted conception: A systematic review of controlled studies. British Medical Journal, 328, 261–265.

K., & Restifo, L. Molecular and comparative genetics of mental retardation. Genetics, 166, 835–881. Jablonski, W. A contribution to the heredity of refraction in human eyes.

Archiv Augenheilk, 91, 308–328. Jacobs, N., Van Gestel, S., Derom, C., Thiery, E., Vernon, P., Derom, R., et al. Heritability estimates of intelligence in twins: Effect of chorion type. Behavior Genetics, 31, 209–217. The g factor. Westport, CT: Praeger. Johnson, W., Bouchard, T. J., Jr., McGue, M., Segal, N.

L., Tellegen, A., Keyes, M., et al. Genetic and environmental influences on the verbal-perceptual-image rotation (VPR) model of the structure of mental abilities in the Minnesota Study of Twins Reared Apart. Intelligence, 35, 452–462.

The Burt affair. New York: Routledge. Juel-Nielsen, N. Individual and environment: Monozygotic twins reared apart. New York: International Universities Press. S., Neale, M.

C., Kessler, R. C., Heath, A. C., & Eaves, L. Parental treatment and the equal environment assumption in twin studies of psychiatric illness. Psychological Medicine, 23, 579–590. Kervinen, K., Kaprio, J., Koskenvuo, M., Juntunen, J., & Kesaniemi, Y. A. Serum lipids and apolipoprotein E phenotypes in identical twins reared apart.

Clinical Genetics, 53, 191–199. Kim, H., Krege, J.

H., Kluckman, K. D., Hagaman, J. R., Hodgin, J. Genetic control of blood pressure and the angiotensinogen locus. Proceedings of the National Academy of Sciences, 92, 2735–2739. L., Holly, A., Iacono, W.

G., McGue, M., & Wilson, L. Physical similarity and twin resemblance for eating attitudes and behaviors: A test of the equal environments assumption. Behavior Genetics, 30, 51–58. S., Moyer, J. S., Jennette, J. C., Peng, L., Hiller, S.K., et al.

Angiotensin-converting-enzyme gene mutations, blood pressures, and cardiovascular homeostasis. Hypertension, 29, 150–157. Kuwata, T., Matsubara, S., Ohkuchi, A., Watanabe, T., Izumi, A., Honma, Y., et al. The risk of birth defects in dichorionic twins conceived by assisted reproductive technology. Twin Research and Human Genetics, 7, 223–227. LaBuda, M., Svikis, D.

S., & Pickens, R. Twin closeness and co-twin risk for substance use disorders: assessing the impact of the equal environment assumption. Psychiatric Research, 70, 155–164. M., & Willerman, L. Modeling IQ change: Evidence from the Texas Adoption Project.

Child Development, 60, 993–1004. C., & Nichols, R. Heredity, environment, and personality: A study of 850 sets of twins. Austin: University of Texas Press. Luciano, M., Smith, G.

A., Wright, M. J., Geffen, G. M., Geffen, L. B., & Martin, N.

On the heritability of inspection time and its covariance with IQ: a twin study. Intelligence, 29, 443–457. Luciano, M., Wright, M. J., & Martin, N. Exploring the etiology of the association between birthweight and IQ in an adolescent twin sample. Twin Research, 7, 62–71. A., & Keith, L.

An atlas of multiple pregnancy: Biology and pathology. New York: Parthenon.

Bachchan Indian Bangla Movie Torrent Download. Causes of variation in birth weight: A study of offspring of twins. Clinial Genetics, 25, 15–24. B., Zyrianova, N.

M., & Kuravsky, L. Longitudinal genetic analysis of childhood IQ in 6- and 7-year-old Russian twins.

Twin Research, 6, 285–291. E., Johansson, B., Berg, S., Pedersen, N.

L., Ahern, F., Petrill, S. Substantial genetic influence on cognitive abilities in twins 80+ years old. Science, 276, 1560–1563.

Big-brained people are smarter: A meta-analysis of the relationship between in vivo brain volume and intelligence. Intelligence, 33, 337–346. McGue, M., Bouchard, T.

J., Jr., Iacono, W. G., & Lykken, D. Behavioral genetics of cognitive ability: A life-span perspective. McClearn (Eds.), Nature, nurture and psychology (pp. Washington, DC: APA Press.

The mating mind: How sexual choice shaped the evolution of human nature. New York: Doubleday. Morris-Yates, A., Andrews, G., Howie, P., & Henderson, S. Twins; A test of the equal environments assumption. Acta Psychiatrica Scandinavica, 81, 322–326. Neisser, U., Boodoo, G., Bouchard, T. J., Jr., Boykin, A.

W., Brody, N., Ceci, S. Intelligence: Knowns and unknowns. American Psychologist, 51, 77–101. B., & Neubaur, A. Nature’s thumbprint: The new genetics of personality. New York: Addison-Wesley.

N., Freeman, F. N., & Holzinger, K. Twins: A study of heredity and environment. Chicago: University of Chicago Press. H., & Patterson, J. The development of the nine-banded armadillo from the primitive streak stage to birth, with especial reference to the question of specific polyembryony. Journal of Morphology, 21, 359.

Oppenheim, J. S., Skerry, J. E., Tramo, M.

J., & Gazzaniga, M. Magnetic resonance imaging of the corpus callosum in monozygotic twins. Annual Neurology Review, 26, 100–104. L., McClearn, G. E., Plomin, R., & Friberg, L. Separated fraternal twins: Resemblance for cognitive abilities.

Behavior Genetics, 15, 407–419. L., McClearn, G.

E., Plomin, R., & Nesselroade, J. Effects of early rearing environment on twin similarity in the last half of the life span. British Journal of Developmental Psychology, 10, 255–267. Pennington, B.

F., Filipek, P. A., Lefly, D., Chhabildas, N., Kennedy, D. N., Simon, J. A twin MRI study of size variations in human brain. Journal of Cognitive Neuroscience, 12, 223–232.

Memories of the Child Development Center study of adopted monozygotic twins reared apart: An unfulfilled promise. Twin Research and Human Genetics, 8, 271–275. Pfefferbaum, A., Sullivan, E. E., & Carmelli, D. Brain structure in men remains highly heritable in the seventh and eighth decades of life.

Neurobiology of Aging, 21, 63–74. The blank slate: The modern denial of human nature.

New York: Viking. Plomin, R., DeFries, J. C., McClearn, G. E., & McGuffin, P. Behavioral genetics (4th ed.). New York: Worth Publishers.

Posthuma, D., De Geus, E. J., Baare, W. S., & Boomsma, D.

The association between brain volume and intelligence is of genetic origin. Nature Neuroscience, 5, 83–84. Posthuma, D., De Geus, E. J., Bleichrodt, N., & Boomsma, D. Twin-singleton differences in intelligence?

Twin Research, 3, 83–87. Fundamentals of human reproduction. New York: McGraw-Hill. Prabhakaran, V., Smith, J. A., Desmond, J.

E., Glover, G. H., & Gabrielli, J. Neuronal substrates of fluid reasoning: An fMRI study of neocortical activation during performance of the Raven’s Progressive Matrices Test. Cognitive Psychology, 33, 43–63. Purcell, S., Eley, T. S., Oliver, B., Petrill, S.

A., Price, T. Comorbidity between verbal and non-verbal cognitive delays in 2-year-olds: A bivariate twin analysis. Developmental Science, 4, 194–207. Reed, T., Carmelli, D., & Rosenman, R. Effects of placentation on selected Type A behaviors in adult males in the National Heart, Lung, and Blood Institute (NHLBI) twin study. Behavior Genetics, 21, 9–19.

A., Finkel, D., Gatz, M., & Pedersen, N. Sources of influence on rate of cognitive change over time in Swedish twins: An application of latent growth models. Experimental Aging Research, 28, 407–433. H., Dolan, C. V., van Baal, C. M., & Boomsma, D.

A twin study of differentiation of cognitive abilities in childhood. Behavior Genetics, 33, 367–381.

A., De Stavola, B. L., & Leon, D. The cognitive cost of being a twin: Evidence from comparisons within families in the Aberdeen children of the 1950s cohort.

British Medical Journal, 331, 1306–1310. J., Harris, E. L., Christian, J. C., & Nance, W. Genetic variance in nonverbal intelligence: Data from the kinships of identical twins.

Science, 205, 1153–1155. The limits of family influence. New York: Guiford Press. Rutherford, S. From genotype to phenotype: Buffering mechanisms and the storage of genetic information.

BioEssays, 22, 1095– 1105. Rutter, M., Thorpe, K., Greenwood, R., Northstone, K., & Golding, J., (2003). Twins as a natural experiment to study the causes of mild language delay: I: Design; twin-singleton differences in language, and obstetric risks. Journal of Child Psychology and Psychiatry, 44, 326–341. Scamvougeras, A., Kigar, D. L., Jones, D., Weinberger, D. R., & Witelson, S.

Size of the human corpus callosum is genetically determined: An MRI study in mono and dizygotic twins. Neuroscience Letters,338, 91–94. Environmental bias in twin studies. Manosevits, G. Lindzey, & D. Thiessen (Eds.), Behavioral genetics: Method and theory (pp. New York: Appleton-Century-Crofts.

Holocaust twins: Their special bond. Psychology Today, 19, 52–58.

Monozygotic and dizygotic twins: A comparative analysis of mental ability profiles. Child Development, 56, 1051– 1058. Twin research perspective on human development. Weisfeld, & C. Weisfeld (Eds.), Uniting psychology and biology: Integrative perspectives on human development (pp. Washington, DC: APA Press.

Entwined lives: Twins and what they tell us about human behavior. New York: Plume. Virtual twins: New findings on within-family environmental influences on intelligence. Journal of Education Psychology, 92, 442–448.

Spotlights (Reared apart twin researchers); research sampling; literature, politics, photography and athletics. Twin Research, 6, 72–81. Indivisible by two: Lives of extraordinary twins. Cambridge, MA: Harvard University Press. More thoughts on the Child Development Center Twin Study. Twin Research and Human Genetics, 8, 276–281. Twins reared apart design.

Howell (Eds.), Encyclopedia of statistics in behavioral science. Chichester, UK: John Wiley & Sons. (In press, 2009). Multiple births: Developmental perspectives. Chicago Companion to the Child, Chicago: University of Chicago Press. L., & Allison, D.

Twins and virtual twins: Bases of relative body weight revisited. International Journal of Obesity, 26, 437–441. L., Chavarria, K. A., & Stohs, J. Twin research: Evolutionary perspective on social relations.

Shackelford & C. Salmon (Eds.), Family relationships: An evolutionary perspective (pp. Oxford, England: Oxford University Press. L., McGuire, S. A., Havlena, J., Gill, P., & Hershberger, S. Intellectual similarity of virtual twin pairs: Developmental trends. Personality and Individual Differences, 42, 1209–1219.

L., Seghers, J. P., Marelich, W. D., Mechanic, M., & Castillo, R. Social closeness of monozygotic and dizygotic twin parents toward their nieces and nephews.

European Journal of Personality, 21, 487–506. Sharma, A., Sharma, V. K., Horn-Saban, S., Lancet, D., Ramachandran, S., & Brahmachari, S. Assessing natural variations in gene expression in humans by comparing with monozygotic twins using microarrays.

Physiological Genomics, 21, 117–123. Monozygotic twins: Brought up apart and together. London: Oxford University Press.

Many little things: One geneticist’s view of complex diseases. Nature Reviews – Genetics, 6, 419–425. Snyderman, M., & Rothman, S. The IQ controversy, the media and publication. New Brunswick, NJ: Transaction. K., Moore, C. J., Williams, C.

J., Reed, T., & Christian, J. Intrapair differences in personality and cognitive ability among young monozygotic twins distinguished by chorion type. Behavior Genetics, 25, 457–466. M., Ronald, A., Harlaar, N., Price, T. S., & Plomin, R.

Phenotypic g early in life: On the etiology of general cognitive ability in a large population sample of twin children aged 2–4 years. Intelligence, 31, 194–210.

Spitz, E., Carlier, M., Vacher-Lavenu, M.-C., Reed, T., Moutier, R., Busnel, M.-C., et al. Long term effect of prenatal heterogeneity among monozygotes.

Current Psychological Cognition, 15, 283–308. Steinmetz, H., Herzog, A., Schlaug, G., Huang, Y., & Jancke, L. Brain (A)symmetry in monozygotic twins. Cerebral Cortex, 5, 296– 300.

Principles of human genetics. San Francisco: W.H. Stromberg, B., Dahlquist, G., Ericson, A., Finnstrom, O., Koster, M., & Stjernqvist, K. Neurological sequelae in children born after in-vitro fertilisation: A population-based study. Lancet, 359, 461– 465. V., Pfefferbaum, A., Swan, G. E., & Carmelli, D.

Heritability of hippocampal size in elderly twin men: Equivalent influences from genes and environment. Hippocampus, 11, 754–762. The IQ game: A methodological inquiry into the heredity-environment controversy. New Brunswick, NJ: Rutgers University Press. M., Cannon, T.

V., Poutanen, V. P., Huttunen, M., et al. Genetic influences on brain structure. Nature Neuroscience, 4, 1–6.

Thorndike, E. Measurement of twins. Journal of Philosophy, Psychology and Scientific Methods, 1, 1–64. Thorpe, K., Greenwood, R., Eivers, A., & Rutter, M. Prevalence and developmental course of ‘secret language.’ International Journal of Language and Communication Disorders, 36, 43–62.

Thorpe, K., Rutter, M., & Greenwood, R. Twins as a natural experiment to study the causes of mild language delay: II: Family interaction risk factors. Journal of Child Psychology and Psychiatry, 44, 342–355. W., & Thompson, P.

Genetics of brain structure and intelligence. Annual Review of Neuroscience, 28, 1–5.

Tomasello, M., Mannle, S., & Kruger, A. Linguistic environment of 1- to 2-year-old twins. Developmental Psychology, 22, 169–176. J., Loftus, W. C., Stukel, T. A., Green, R. L., Weaver, J.

B., & Gazzaniga, M. Brain size, head size, and intelligence quotient in monozygotic twins. Neurology, 50, 1246–1252. Trejo, V., Derom, C., Vlietinck, R., Ollier, W., Silman, A., Ebers, G., et al. X chromosome inactivation patterns correlate with fetal-placental anatomy in monozygotic twin pairs: Implications for immune relatedness and concordance for autoimmunity.

Molecular Medicine, 1, 62–70. A., Moffit, T. B., & Caspi, A. Maternal adjustment, parenting and child behavior in families of school-aged twins conceived after IVF and ovulation induction. Journal of Child Psychology and Psychiatry, 44, 316–325.

Turkheimer, E., Haley, A., Waldron, M., D’Onofrio, B., & Gottesman, I. Socioeconomic status modified heritability of IQ in young children. Psychological Science, 14, 623–628. Vandenberg, S. Contributions of twin research to psychology.

Manosevits, G. Lindzey, & D. Thiessen (Eds.), Behavioral genetics: Method and theory (pp. New York: Appleton- Century-Crofts. Van Ijzendoorn, M.

H., Juffer, F., & Poelhuis, C. Adoption and cognitive development: A meta-analytic comparison of adopted and nonadopted children’s IQ and school performance. Psychological Bulletin, 131, 301–316. Waddington, C. Genetic assimilation of an acquired character.

Evolution, 7, 118–126. Waddington, C. The strategy of the genes.

New York: Macmillan. Wainwright, M. A., Wright, M.

I., Geffen, G. M., Luciano, M., & Martin, N. The genetic basis of academic achievement on the Queensland Core Skills Test and its shared genetic variance with IQ. Behavior Genetics, 35, 133–145. G., Cervoni, N., Champagne, F.

A., D’ A lessio, A. C., Sharma, S., Seckl, J. Epigenetic programming by maternal behavior.

Nature Neuroscience, 7, 847–854. Beitrage zur physiologie und pathologie der mehrlingsgeburten beim menschen. Pflugers Archiv fur die Gesamte Physiologie de Menschen und der Tiere, 88, 346–430.

Twins: Genetic influence on growth. Bouchard (Eds.), Sports and human genetics (pp. Champaign, IL: Human Kinetics.

Twin growth: Initial deficit, recovery, and trends in concordance from birth to nine years. Human Biology, 6, 205–220.

The Louisille Twin Study: Developmental synchronies in behavior. Child Development, 54, 298–316. C., Gottesman, I. I., & Petronis, A. Phenotypic differences in genetically identical organisms: The epigenetic perspective. Human Molecular Genetics, 14, R11–R18. L., Lidsky, A.

S., Guttler, F., Chandra, T., & Robson, K. Cloned human phenylalanine hydroxylase gene allows prenatal diagnosis and carrier detection of classical phenylketonuria. Nature, 306, 151–155. P., Miles, M.

F., Kendler, K. S., Jackson-Cook, C., Bowman, M. L., & Eaves, L. Epistatic and environmental control of genome-wide gene expression. Twin Research, 8, 5–15. The focus of the present chapter is on what twin studies have thus far contributed to our understanding of individual differences in intelligence.

A modern approach to programming for the Erlang VM. Elixir is a programming language built on top of the Erlang VM. As Erlang, it is a functional language built to support distributed, fault-tolerant, non-stop applications with hot code swapping. Elixir is also dynamic typed but, differently from Erlang, it is also homoiconic, allowing meta-programming via macros.

Elixir also supports polymorphism via protocols (similar to Clojure’s), dynamic records and provides a reference mechanism. Finally, Elixir and Erlang share the same bytecode and data types. This means you can invoke Erlang code from Elixir (and vice-versa) without any conversion or performance hit. This allows a developer to mix the expressiveness of Elixir with the robustness and performance of Erlang. Source: Posted in. « Muslims that arrive here do not even believe that this country belongs to us, to the white man. » Eli Yishai, Interior Minister of Israel. Israel enacts law allowing authorities to detain illegal migrants for up to 3 year Until now, migrants caught by IDF have been transferred to the Saharonim detention facility in the south; Interior Minister says migrants do not recognize that Israel ‘belongs to the white man.’ A law granting Israeli authorities the power to detain illegal migrants for up to three years came into effect on Sunday, in the wake of widening public controversy over the influx of African migrants who cross into Israel along its border with Egypt.

The law makes illegal migrants and asylum seekers liable to jail, without trial or deportation, if caught staying in Israel for long periods. In addition, anyone helping migrants or providing them with shelter could face prison sentences of between five and 15 years. The law amended the Prevention of Infiltration Law of 1954, passed to prevent the entry of Palestinians as part of emergency legislation. The law is expanded to address migrant workers or asylum seekers who enter Israel without posing a threat to Israel’s security. According to the law, migrant workers already here could be jailed for the most minor offense such as spraying graffiti or stealing a bicycle – infractions for which they would not have been detained before. So far, all migrants who have been caught by the Israel Defense Forces on the Israel-Egypt border have been transferred to the Saharonim detention facility which holds 2,000 spaces.

The facility is currently being expanded to 5,400. The Interior Ministry has reported that they are implementing the amendment and will fill up Saharonim, where they will be held until the ministry « finds other solutions. » According to the Interior Ministry, the Saharonim detention center will run out of space within a month. All those detained go through an identification process and a medical examination. Those who file for asylum receive a temporary visa to remain in Israel. Sudanese and Eritreans, however, are not allowed to file for asylum, although they are automatically eligible for temporary shelter and a one-way ticket to Tel Aviv. Some migrants continue independently to Arad or Eilat where they often have acquaintances. According to the ministry, up to 60,000 African migrant currently live in Israel, with 2,031 entering in the month of May alone.

Human rights organizations see the amendment as a harsh step which contradicts the United Nations Convention Relating to the Status of Refugees (CRSR). According to the Hotline for Migrant Workers, the law was « born in sin » and is a « dark moment for Israel. » « Instead of acting like all civilized countries and verifying requests for asylum and granting refugee status to those who are eligible, which Israel is obligated to do under the UN convention, the state sees mass imprisonment of thousands of people, women and children, whose only offense was seeking escape from murderous regimes, as a solution to the problem. This solution will not solve a thing as it is neither humane nor effective. In a statement, the Israel Prison Services said that it was ready to « take in as many illegal residents as come to its facilities, with the required detainment authority and according to time of detention. » « For this purpose, several wards outside Saharonim have been converted, and we will prepare according to need, » the statement added.

Meanwhile on Sunday, Israeli daily Maariv published an interview with Interior Minister Eli Yishai, in which he stated that most of the « Muslims that arrive here do not even believe that this country belongs to us, to the white man. » « I will continue the struggle until the end of my term, with no compramises, » Yishai continued, stating that he would use « all the tools to expel the foreigners, until not one infiltrator remains. » By Source: Posted in Navigation des articles.

Label Templates Easily download free 8.5' x 11' label templates for laser and inkjet printing. We offer a complete collection of label templates including popular CD label templates, standard template sizes similar in layout to Avery®, as well as templates for address, mailing, shipping, round, audio cassette, VHS and diskettes.

PDF templates (viewable using Adobe Acrobat Reader) – These template formats can be used in graphic programs such as Adobe Illustrator, Photoshop, InDesign, Quark Express and several others. If you plan to print out the template you’ll need to uncheck 'fit to page' in the print options or the image will be smaller than actual size. For Palm users, please use PDF files.

Requirements: Acrobat Reader for Palm.