What Virtual Twins Reveal About General Intelligence and Other Behaviors

The Fullerton Virtual Twin Study has been assessing the behaviors of an unusual sibship since 1982. Virtual twins (VTs) are same-age, unrelated siblings reared together since infancy. They replicate the rearing situation of twins but without the genetic link, enabling direct assessment of shared environmental effects on behavior. An updated analysis of IQ data, based on an increased sample of 142 VT pairs (7.87 years, SD=8.22), is presented. Intraclass correlations of .28 (IQ) and .11 (subtest profile) indicated modest shared environmental influences on intelligence. Findings from the Twins, Adoptees, Peers and Siblings (TAPS) project that studies virtual twins and other kinships are described.     

IQ correlations in transracial adoptive families

IQ tests were administered to all available members over 4 years old in 101 transracial adoptive families when the adopted children were an average of 7 years old and again when they averaged 17 years old. At both times, 426 members of 93 families were studied; 398 were seen in person and administered the WAIS-R or WISC-R. IQ correlations were calculated for adopted and biological parent-child pairs, and for genetically related and unrelated siblings. Educational levels of birth parents were correlated with the IQ scores of their adopted-away children. Results show that biologically related family members tended to resemble each other intellectually more than did adoptive family members at both time points. IQ correlations for biological parent-child pairs exceeded those for adoptive parent-child pairs, and correlations were greater for genetically related than unrelated siblings. In late adolescence, the IQ scores of unrelated siblings in the transracial adoptive families were more similar than those of unrelated adolescent siblings pairs reported in other studies. The pattern of IQ correlations for unrelated siblings suggested that familial environmental influences on IQ decline from childhood to late adolescence, but this conclusion was not supported by parent-child IQ correlations. The effects of selective placement on familial IQ correlations were small. Estimates of genetic and familial environmental influences on IQ were very similar to those of other studies. This suggests that the influences on intellectual development in this sample of black/interracial adoptees reared in white families are similar to those for children in the majority populations of the United States and Western Europe.     

Genetic and environmental covariance structures among WISC-R subtests: A twin study

Genetic and environmental correlations among 11 Wechsler Intelligence Scale for Children-Revised (WISC-R) subtests were estimated from a sample of 143 twin pairs using the methodology of multivariate behavioral genetics. The genetic correlations among subtests varied in magnitude and exhibited a pattern similar to corresponding phenotypic correlations. Correlations due to within-pair environmental influences were generally small, whereas those due to shared environmental influences tended to be intermediate. Several hypotheses regarding the structures of the additive-genetic, common environmental, and within-pair environmental covariance matrices were tested and evidence for differential factor structures was obtained. A model hypothesizing a three-factor structure for the genetic covariance matrix, a single factor for the common environmental matrix, and a single factor plus specifics for the within-pair environmental matrix fit the data well. Thus, the three-factor phenotypic structure typically observed in Wechsler Intelligence Scale for Children (WISC) data may be due largely to genetic influences.     

Multivariate analysis of cognitive and temperament measures in 24-month-old adoptive and nonadoptive sibling pairs

The genetic and environmental etiologies of covariation among measures of temperament (the Bayley Infant Behavior Record's Affect and Task Orientation Scales and the first principal component from the New York Longitudinal Study's difficult temperament items) and cognition (the Bayley Mental Development Index [MDI] and the total score on the Sequenced Inventory of Communication Development [SICD] were assessed at 24 months of age in 70 biologically related and 66 unrelated pairs of siblings in the Colorado Adoption Project. Between- and within-pair mean cross products were equated to expectations using the LISREL multiple-group specification (Fulker, Baker and Bock, 1983) to obtain maximum-likelihood estimates of genetic and environmental factor loadings and specific variances. The full one-factor model, with one general factor and five specific factors, provides a satisfactory fit to the data (χ² = 27.8, df = 30, P = 0.58). Genetic influences are important for the Bayley MDI and SICD languages measures, but not for temperament measures. Little or no evidence was found for shared-sibling environmental influences for any of the measures. Various hypotheses regarding genetic and environmental correlation structures were also tested.     

Herrnstein''s syllogism: genetic and shared environmental influences on IQ, education, and income

Genotypes may influence the phenotypic associations among IQ, education, and income. To investigate this hypothesis, we believe that the appropriate methodology requires estimation of genetic and environmental influences using data able to separate these influences. The National Longitudinal Survey of Youth (NLSY) is a nationally representative sample that contains genetically-informative full- and half-siblings (28–35 years old in 1992; Ns=1943 full-siblings, 129 half-siblings). A biometric genetic model was fit that estimated the shared environmental and genetic variance components of IQ, years of education, and hourly income. The total heritabilities were 0.64 for IQ, 0.68 for education, and 0.42 for income. Heritabilities due to a common genetic factor were 0.35 for IQ, 0.52 for education, and 0.12 for income. Environmental influences due to a common shared environmental factor were 0.23 for IQ, 0.18 for education, and 0.08 for income. The model predicted a correlation of 0.63 between IQ and education and 0.34 between IQ and income. Sixty-eight percent of the former and 59% of the latter was genetically mediated; the remainder was mediated by common shared environment. These findings suggest that social inequality in the United States has its origin in both genetically-based traits and in different environmental backgrounds.     

Common Genetic and Environmental Influences on Major Depressive Disorder and Conduct Disorder

The evidence for common genetic and environmental influences on conduct disorder (CD) and major depressive disorder (MDD) in adolescents was examined. A sample of 570 monozygotic twin pairs, 592 dizygotic twin pairs, and 426 non-twin siblings, aged 12-18 years, was recruited from the Colorado Twin Registry. For the past year data, there was a significant correlation between the genetic influences on MDD and CD and, for the lifetime data, there was a significant correlation between the genetic influences on MDD and CD, and a significant correlation between the nonshared environmental influences on MDD and CD. Our results suggest that some genetic factors will increase an individual's vulnerability to both MDD and CD in adolescence.     

Genetic and environmental influences on individual differences in printed word recognition

The genetic and environmental etiologies of individual differences in printed word recognition and related skills were explored in 440 identical and fraternal twin pairs between 8 and 18 years of age. A theoretically driven measurement model identified five latent variables: IQ, phoneme awareness, word recognition, phonological decoding, and orthographic coding. Cholesky decomposition models on these five latent constructs revealed the existence of both common and independent genetic effects, as well as non-shared environmental influences. There was evidence for moderate genetic influences common between IQ, phoneme awareness, and word-reading skills, and for stronger IQ-independent genetic influences that were common between phoneme awareness and word-reading skills, particularly phonological decoding. Phonological and orthographic coding skills in word recognition had both significant common and significant independent genetic influences, with implications for "dual-route" and "connectionist" reading models, subtypes of reading disabilities, and the remediation of reading disabilities.     

The genetic and environmental architecture to the stability of IQ: Results from two independent samples of kinship pairs

Existing research has revealed that IQ remains relatively stable over the life course, though questions remain about how stable IQ is and whether the stability of IQ varies across different developmental periods of the life course. Despite this stability, there are also questions surrounding the factors that might explain the stability of IQ. Against this backdrop, we conduct bivariate genetic models to estimate genetic, shared environmental, and nonshared environmental influences on the stability of IQ. To do so, we analyze kinship pairs drawn from two separate longitudinal samples: The National Collaborative Perinatal Project (CPP) and the National Longitudinal Study of Adolescent Health (Add Health). Across both samples, IQ was found to be relatively stable. Moreover, the genetic analyses revealed that between 66% and 83% of the stability in IQ was due to genetic factors and between 43% and 69% of the change in IQ was due to genetic factors. The remainder of the stability and change in IQ was the result of a combination of shared and nonshared environmental influences. Importantly, some substantive race differences emerged in respect to genetic and environmental influences on the stability of IQ. We conclude with a discussion of the limitations of the study and avenues for future research.     

The structure of genetic influences on general cognitive, language, phonological, and reading abilities

The etiology of individual differences in literacy, phonological awareness, and language ability is reported in 126 pairs of monozygotic and dizygotic twins. At age 6 and 7 years, more than 60% of the variance in literacy was heritable. Heritabilities for 6- and 7-year-olds were .52 and .62, respectively, for phonological awareness and .43 and .50, respectively, for language ability. After genetic effects on IQ were controlled, a separate genetic influence was identified that acted on literacy, phonological awareness, and language. No genetic link between phonological awareness and literacy independent of general language ability was found; such covariance was mediated through environmental influences. Individual differences in literacy ability are substantially influenced by genetic factors, some of which also act on phonological awareness and general language ability.     

Genetic influences on the overlap between low IQ and antisocial behavior in young children

The well-documented relation between the phenotypes of low IQ and childhood antisocial behavior could be explained by either common genetic influences or environmental influences. These competing explanations were examined through use of the Environmental Risk Longitudinal Twin Study 1994-1995 cohort (Moffitt & the E-Risk Study Team, 2002) of 1,116 twin pairs and their families. Children's IQ was assessed via individual testing at age 5 years. Mothers and teachers reported on children's antisocial behavior at ages 5 and 7 years. Low IQ was related to antisocial behavior at age 5 years and predicted relatively higher antisocial behavior scores at age 7 years when antisocial behavior at age 5 years was controlled. This association was significantly stronger among boys than among girls. Genetic influences common to both phenotypes explained 100% of the low IQ-antisocial behavior relation in boys. Findings suggest that specific candidate genes and neurobiological processes should be tested in relation to both phenotypes.     

Genetic Influence on the Variance in Coincidence Timing and its Covariance with IQ: A Twin Study

Performance measures on a coincidence timing task have previously been associated with psychometric IQ suggesting that the ability of an individual to devote processing resources at the required time may account for some of intelligence test variance. Using the twin design, this study investigates whether genetic variability explains some of the variance in coincidence timing and whether common genetic factors account for the association with intellectual functioning. Fifty-five 16-year old twin pairs (28 MZ, 27 DZ) were tested. Individual differences in number of hits (HITS), mean absolute error (MAE), and intra-individual trial-to-trial consistency (SD) were significantly influenced by genetic factors, accounting for approximately 50 percent of the variance. The correlation between coincidence timing and psychometric IQ was confirmed and ranged from 0.11 to 0.53 with a mean correlation of 0.33. In the limited sample, the correlation between IQ and the coincidence timing measures appeared to be mediated largely by a unique environmental factor, with only a small loading of SD on the genetic factor influencing IQ. However, as the confidence intervals in the other genetic cross loadings are large, we cannot exclude the possibility of a much stronger genetic influence.     

Investigating cognitive transfer within the framework of music practice: genetic pleiotropy rather than causality

The idea of far transfer effects in the cognitive sciences has received much attention in recent years. One domain where far transfer effects have frequently been reported is music education, with the prevailing idea that music practice entails an increase in cognitive ability (IQ). While cross‐sectional studies consistently find significant associations between music practice and IQ, randomized controlled trials, however, report mixed results. An alternative to the hypothesis of cognitive transfer effects is that some underlying factors, such as shared genes, influence practice behaviour and IQ causing associations on the phenotypic level. Here we explored the hypothesis of far transfer within the framework of music practice. A co‐twin control design combined with classical twin‐modelling based on a sample of more than 10,500 twins was used to explore causal associations between music practice and IQ as well as underlying genetic and environmental influences. As expected, phenotypic associations were moderate (r = 0.11 and r = 0.10 for males and females, respectively). However, the relationship disappeared when controlling for genetic and shared environmental influences using the co‐twin control method, indicating that a highly practiced twin did not have higher IQ than the untrained co‐twin. In line with that finding, the relationship between practice and IQ was mostly due to shared genetic influences. Findings strongly suggest that associations between music practice and IQ in the general population are non‐causal in nature. The implications of the present findings for research on plasticity, modularity, and transfer are discussed.     

An extension of construct validity for personality scales using twin-based criteria

A twin-based analysis of personality inventory scales yields information relevant to construct validity. If scales validly measure dimensions influenced developmentally either by environmental factors common to cotwins or by genetic similarities, these influences should be apparent in patterns of intraclass correlations and within-pair variance ratios for monozygotic and dizygotic pairs. Criteria for construct validity derived from adolescent twin statistics are illustrated with Wiggins' MMPI Content Scales. A type of discriminant validity indicates that some scales are not distinct in terms of the influences of familial similarity. Sex differences are examined.     

Socioeconomic status and school grades: Placing their association in broader context in a sample of biological and adoptive families

SES has long interested researchers investigating school achievement. Its effects are often addressed by studying predictors of achievement in economically disadvantaged samples living primarily in biological families, confounding genetic and environmental influences. Little is known about SES's purely environmental effects. We measured them in 617 adoptive and biological families, adjusting for sample restriction of SES range. Controlling for gender, parenting, parental expectations for educational attainment (PEEA), IQ, engagement in school, and genetic and shared environmental influences on sibling pairs, SES still made a small but significant nonshared environmental contribution to school grades. IQ, PEEA, and SES had collinear associations with school grades, as did engagement and parenting. The associations of IQ and engagement with school grades were largely independent of each other. The link between PEEA and IQ was stronger in adoptive than biological offspring. We discuss the implications of these findings.     

Genetics and intelligence: Recent data

During the last three years, cognitive data have been reported for 4,639 pairings using the family design, 2,540 pairings using the adoption design, and 2,164 pairs of twins. Together, these extensive data point to less heritable influence on IQ than is indicated by the widely cited older data. The difference between the newer and older data could be due to environmental or genetic secular changes in the population or to methodological differences. Contrary to usual assumptions, the newer data also suggest that more of the environmental influences for parents and offspring and for older siblings operate within families (making family members different from one another) than between families (making family members similar to one another).     

Why do measures of normal and disordered personality correlate? A study of genetic comorbidity

The genetic and environmental correlations between measures of normal (NEO-FFI) and abnormal personality (Dimensional Assessment of Personality Pathology: DAPP-BQ) were estimated in a sample of 545 volunteer general population twin pairs (269 monozygotic and 276 dizygotic pairs). The largest genetic correlations were observed between the 18 DAPP-BQ dimensions and NEO-FFI neuroticism (range = .05 to .81; median = .48), extraversion (range = -.65 to .33; median = -.28), agreeableness (range = -.65 to .00; median = -.38), and conscientiousness (range = -.76 to .52; median = -.31). The smallest genetic correlations were found between the DAPP-BQ dimensions and NEO-FFI openness (range = -.17 to .20; median = -.04). The environmental correlations are lower in magnitude but show the same pattern of correlations between DAPP-BQ and NEO-FFI scales. These results indicate that these two scales share a common broad-based genetic architecture, whereas the environmental influences show greater scale specificity.     

More Than Just IQ: School Achievement Is Predicted by Self-Perceived Abilities—But for Genetic Rather Than Environmental Reasons

ABSTRACT— Evidence suggests that children's self-perceptions of their abilities predict their school achievement even after one accounts for their tested cognitive ability (IQ). However, the roles of nature and nurture in the association between school achievement and self-perceived abilities (SPAs), independent of IQ, is unknown. Here we reveal that there are substantial genetic influences on SPAs and that there is genetic covariance between SPAs and achievement independent of IQ. Although it has been assumed that the origins of SPAs are environmental, this first genetic analysis of SPAs yielded a heritability of 51% in a sample of 3,785 pairs of twins, whereas shared environment accounted for only 2% of the variance in SPAs. Moreover, multivariate genetic analyses indicated that SPAs predict school achievement independently of IQ for genetic rather than environmental reasons. It should therefore be possible to identify "SPA genes" that predict school achievement independently of "IQ genes."     

IQ and social class

A brief overview is given of the nature-nurture debate elicited on the basis of Arthur Jensen's article concerning group differences in IQ test results published in 1969.Sandra Scarr-Salapatek found that the contribution of genetic variance in IQ test results tended to be larger in a more stimulating environment and proportionately smaller in a disadvantaged environment. This finding has been tested on data from a Swedish twin study. A comparison of within-pair similarity for MZ and DZ twins with varying social background has been made. Genetic variance seems to account for a greater proportion of the variance in IQ test scores in social group I and II compared to social group III. This is in accordance with the conclusions drawn by Scarr-Salapatek and thus tend to support the above-mentioned hypothesis. It therefore seems as though the potentially restricting effects of working class conditions for the development of the kind of abilities measured by conventional IQ tests would tend to reduce the degree to which genetic differences are expressed. More stimulating environmental opportunities, on the other hand, tend to allow a greater range of such differences in the middle and higher social classes.     

Reading performance and general cognitive ability in twins with reading difficulties and control pairs

The etiology of the observed relationship between general cognitive ability and reading performance was investigated by analyzing data from samples of twin pairs tested in the Colorado Learning Disabilities Research Center. Bivariate phenotypic and genetic structural equation models were fitted to data from 486 twin pairs selected for reading deficits (276 identical and 210 same-sex fraternal) and 314 control pairs (195 identical and 119 same-sex fraternal). Subtests of the Peabody Individual Achievement test (Reading Recognition, Reading Comprehension, and Spelling) were used as measures of reading performance, and verbal and performance IQ scores from the Wechsler Intelligence Scale for Children-Revised (WISC-R) or Wechsler Adult Intelligence Scale-Revised (WAIS-R) were used as indices of general cognitive ability. The results of these confirmatory factor analyses indicate that shared environmental influences do not contribute significantly to covariation between general cognitive ability and reading performance. In contrast, genetic influences contribute substantially to the relationship between the two latent factors. Moreover, the genetic and phenotypic variances of reading performance in the proband group are larger than those in controls, whereas both the genetic and phenotypic correlations between reading performance and general cognitive ability are lower in probands. These results are consistent with recent reports that reading disability may be caused by one or more genes with major effects.     

Sex differences in genetic and environmental influences on DSM-III-R attention-deficit/hyperactivity disorder

Approximately 5% of children are affected by attention-deficit/hyperactivity disorder (ADHD), and more boys are affected than girls. This study examined the magnitude of genetic and environmental influences on ADHD and several questions regarding sex differences in its prevalence and liability. The participants were 2,391 twin and sibling pairs from Australia, ages 3-18. ADHD symptoms in the general population were highly heritable (h2 = .85-.90), as were deviant ADHD scores in the selected population. The magnitude of familial influences was similar for boys and girls, although there were shared environmental influences on ADHD in girls but not boys and dominance genetic influences on ADHD in boys but not girls. Specific genetic and environmental influences were highly similar for boys and girls. Evidence supported the polygenic multiple threshold model rather than the constitutional variability model of sex differences in ADHD.