Heritability × SES Interaction for IQ: Is it Present in US Adoption Studies?

An interaction between socioeconomic status (SES) and the heritability of IQ, such that the heritability of IQ increases with higher SES, has been reported in some US twin studies, although not in others, and has generally been absent in studies outside the US (England, Europe, Australia). Is such an interaction present in US adoption studies? Data from two such studies, the Texas and the Colorado Adoption Projects, were examined, involving 238-469 adopted children given IQ tests at various ages. A mini multi-level analysis was made of the prediction of the IQs by the SES of the rearing home (a composite of parental education and occupation), by the birth mother's intelligence, and by the interaction of the two. Neither study showed any substantial heritability × SES interaction: the effect size estimates in units comparable to twin moderation models were negative (- 0.042 and - 0.004), and the meta-analytic estimate for the combined analysis was - 0.27 (SE = 0.042) with a 95% confidence interval of - 0.109 to 0.054. Thus, while we cannot rule out positive moderation based on our two studies, the joint agreement across these studies, and with the non-US twin studies, warrants attention in further research. SES may not fully capture proximal familial-environmental aspects that moderate child IQ.

What Can an Adoption Study Tell Us About the Effect of Prenatal Environment on a Trait?

Adoption studies provide possibilities for estimating the extent to which prenatal environmental events account for individual differences on a trait. Correlations with birth mothers but not adoptive mothers suggest the presence of genetic or prenatal environmental effects; higher correlations with birth mothers than with birth fathers suggest the presence of the latter. Changes over time may also be relevant. The concepts involved are illustrated with parent-child IQ correlations from the Texas and Colorado Adoption Projects.

Is there a genetic correlation between general factors of intelligence and personality?

We tested a hypothesis that there is no genetic correlation between general factors of intelligence and personality, despite both having been selected for in human evolution. This was done using twin samples from Australia, the United States, the Netherlands, Great Britain, and Croatia, comprising altogether 1,748 monozygotic and 1,329 same-sex dizygotic twin pairs. Although parameters in the model-fitting differed among the twin samples, the genetic correlation between the two general factors could be set to zero, with a better fit if the U.S. sample was excepted.

The genetic correlation between procrastination and impulsivity.

The reported genetic correlation of 1.0 between the traits of procrastination and impulsivity (Gustavson, D. E., Miyake, A., Hewitt, J. K., & Friedman, N. P. (2014). Psychological Science), which was held to support an evolutionary origin of the relationship between the two traits, was tested in data from two large samples of twins from Australia. A genetic correlation of 0.299 was obtained. It was concluded that, although the presence of a genetic correlation between the two traits was supported, the modest magnitude of the correlation was such as to be consistent with many possible hypotheses, evolutionary and otherwise, about causal relationships between the traits in question.

How Much Is Personality Structure Affected If One or More Highest-Level Factors Are First Removed? A Sequential Factors Approach.

Rotations of 1 to 12 factors were compared by Goldberg's "bass-ackward" method, with or without initially holding constant one or more principal components. Two sets of data were employed: ratings by 320 undergraduates using 435 personality-descriptive adjectives, and 512 Oregon community members' responses to 184 scales from 8 personality inventories. Holding constant none or one or three initial factors made relatively little difference to the resulting structure. On the whole, that structure was not strongly hierarchical: allowing an additional dimension usually resulted in a new substantive dimension rather than in the splitting of an old one.

Do Personality Traits Conform to Lists or Hierarchies?

Are personality traits mostly related to one another in hierarchical fashion, or as a simple list? Does extracting an additional personality factor in a factor analysis tend to subdivide an existing factor, or does it just add a new one? Goldberg's "bass-ackwards" method was used to address this question, based on rotations of 1 to 12 factors. Two sets of data were employed: ratings by 320 undergraduates using 435 personality-descriptive adjectives, and 512 Oregon community members' responses to 184 scales from 8 personality inventories. In both, the view was supported that personality trait structure tends not to be strongly hierarchical: allowing an additional dimension usually resulted in a new substantive dimension rather than in the splitting of an old one, and once traits emerged they tended to persist.

The differential heritability of personality item clusters.

An earlier twin analysis by Loehlin and Nichols was repeated with a considerably larger twin sample (2,600 adult Australian twin pairs). Like the previous analysis, it was based on item clusters, although from different questionnaires. Unlike the previous study, it found consistent differences in MZ-DZ discrepancy in correlation among the clusters. A more elaborate analysis confirmed this for the clusters, but not for broad Extraversion and Neuroticism composites based upon them.

Is there an active gene-environment correlation in adolescent drinking behavior?

A scale based on alcohol-related behaviors and an item on shared friends from the National Merit Twin Study were used in an attempt to confirm the finding of Cleveland et al. (1995, J Genet Psychol 166:153-169) of gene-environment correlation in adolescents' drinking behavior, a correlation based on the differential selection of peers. Results from samples of 490 MZ and 336 same-sex DZ pairs were consistent in direction with the hypothesis, although quantitatively modest. This consistency appeared, however, to depend entirely on the female twins in the sample.

The effect of assumptions about parental assortative mating and genotype-income correlation on estimates of genotype-environment interaction in the National Merit Twin Study.

In a previous paper (Harden et al. in Behav Gen 37:273-283, 2007) models of genotype-environment interaction were fitted to data from the National Merit Twin Study, resulting in evidence for an interaction: the heritability of National Merit Qualifying Test scores increased at higher levels of family income. The present paper investigates two assumptions made in the previous modeling. These were a lack of resemblance between parents for cognitive skill, and possible correlations between family income and a child's genes because of the contribution of parental genes to both. The assumptions were found not to seriously affect estimates of the interaction effect-heritability still increased with income-but they did make a difference for other parameter estimates from the modeling. One possible explanation of the observed interaction, decreasing levels of assortative mating at higher income levels, was examined and found not to be consistent with other evidence from the study. Another possible explanation, a greater freedom of members of DZ pairs at higher income levels to follow independent interests, remained plausible.

Relative finger lengths, sex differences, and psychological traits.

Various finger length and personality and ability measures were obtained for a sample of Australian adolescent twins (306 boys and 397 girls). A new measure of relative finger length (the length of a given finger relative to the sum of all four fingers) was investigated, and shown to be superior to the traditional 2D:4D for discriminating between the sexes. It also had the advantage of permitting a more analytic approach: for example, the 2nd finger-length contributed much more than the 4th finger length to the sex difference in 2D:4D, and a smooth gradient of sex differences across the hand was evident. Sex differences on right hands were greater than those for left hands. Within-sex correlations were obtained between the various finger-length measures and a personality and an ability scale that showed relatively large sex differences (Eysenck's Psychoticism scale and the spatial subscale from Jackson's Multidimensional Aptitude Battery). The correlations were low, but on the whole consistent with the between-sex differences for the girls. For the boys, this was so for Psychoticism, but spatial ability was, if anything, correlated in the opposite direction.

Antecedents of children's adult outcomes in the Texas Adoption Project.

Four composite variables concerning life outcomes were derived from a brief mail questionnaire describing 478 adults, now in their 30s and 40s, who had participated as children in the Texas Adoption Project. Responses were obtained from the participants themselves, their parents, and their siblings. MMPI scores of the parental generation were correlated with the adult outcomes of their biologically related and unrelated children. The obtained correlations were low, but for the biological relationships positive parent adjustment went with positive life outcomes of their children, whereas for adoptive relationships the reverse was the case. Favorable MMPI scores from late adolescence were favorably related to adult outcomes, as were favorable personality ratings from childhood.

Males do not reduce the fitness of their female co-twins in contemporary samples.

Lummaa et al. (2007) presented historical data collected from twins born in Finland between 1734 and 1888 which suggested that females (N = 31) born as part of an opposite sex (OS) twin pair were 25% less likely to reproduce than female twins (N = 35) born as part of a same sex (SS) pair. They hypothesized that this reduction in fitness was due to masculinization of the female fetus via prenatal effects of the hormones of a male fetus. Because such masculinization would presumably take place in modern populations as well, it would seem important to establish to what degree it does so, and if so, whether reproduction is affected. We therefore address the question of reproduction differences in individual female twins from same-sex (N = 1979) and opposite-sex (N = 913) dizygotic pairs in studies carried out in Australia, the Netherlands, and the United States. In all three samples, there were no differences in the number of children or age of first pregnancies in women from same sex pairs compared to those from opposite sex pairs. Similarly, there were no differences in psychological femininity between women from pairs of the same or opposite sex.

Multivariate genetic analyses of the 2D:4D ratio: examining the effects of hand and measurement technique in data from 757 twin families.

The ratio of the lengths of the second to fourth digits of the hand (2D:4D) is a sexually dimorphic trait that has been proposed as a measure of prenatal testosterone exposure and a putative correlate of a variety of later behavioral and physiological outcomes including personality, fitness and sexual orientation. We present analyses of 2D:4D ratios collected from twins (1413 individuals) and their nontwin siblings (328 individuals) from 757 families. In this sample 2D:4D was measured from photocopies using digital calipers, and for a subset of participants, computer-aided measurement. Multivariate modeling of the left- and right-hand measurements revealed significant genetic and environmental covariation between hands. The two methods yielded very similar results, and the majority of variance was explained by factors shared by both measurement methods. Neither common environmental nor dominant genetic effects were found, and the covariation between siblings could be accounted for by additive genetic effects accounting for 80% and 71% of the variance for the left and right hands, respectively. There was no evidence of sex differences in the total variance, nor in the magnitude or source of genetic and environmental influences, suggesting that X-linked effects (such as the previously identified association with the Androgen receptor) are likely to be small. However, there were also nonshared environmental effects specific to each hand, which, in addition to measurement error, may in part explain why some studies within in the literature find effects for the 2D:4D ratio of one hand but not the other.

Genetic and environmental influences on adult life outcomes: evidence from the Texas Adoption Project.

A short mail questionnaire was sent to individuals, now adults, who had been studied over 30 years ago as children in the Texas Adoption Project. Their parents and (in many cases) siblings also described them using the same questionnaire, and the parents described themselves as well. The questionnaire was designed to obtain information about educational, occupational, and marital outcomes, as well as adult problems and personality. Results were obtained for 324 adopted and 142 biological children from the original 300 families, and for 266 parents. Although both the adopted and biological offsprings' outcomes were generally positive, those for the adopted offspring were somewhat less so. Biologically related family members tended to be more similar in their life outcomes than biologically unrelated family members, suggesting that genes were playing an important role.

Genotype by environment interaction in adolescents' cognitive aptitude.

In a replication of Turkheimer, Haley, Waldron, D'Onofrio, Gottesman II (2003, Socioeconomic status modifies heritability of IQ in young children. Psychological Science, 14:623-628), we investigate genotype-environment (G x E) interaction in the cognitive aptitude of 839 twin pairs who completed the National Merit Scholastic Qualifying Test in 1962. Shared environmental influences were stronger for adolescents from poorer homes, while genetic influences were stronger for adolescents from more affluent homes. No significant differences were found between parental income and parental education interaction effects. Results suggest that environmental differences between middle- to upper-class families influence the expression of genetic potential for intelligence, as has previously been suggested by Bronfenbrenner and Ceci's (1994, Nature-nurture reconceptualized in developmental perspective: a bioecological model Psychological Review, 101:568-586) bioecological model.

Population differences in finger-length ratios: ethnicity or latitude?

The relative length of the second and fourth fingers (the 2D:4D ratio) has been taken to be an indicator of prenatal exposure to testosterone, and hence possibly relevant to sexual orientation and other sex-differentiated behaviors. Studies have reported a difference in this ratio between Caucasian males in Britain and in the U.S.: higher average 2D:4D ratios were obtained in Britain. This raises the question of whether differences among different Caucasian gene pools were responsible or whether some environmental variable associated with latitude might be involved (e.g., exposure to sunlight or different day-length patterns). This question was explored by examining 2D:4D ratios for an Australian adolescent sample. The Australians were predominantly of British ancestry, but lived at distances from the equator more like those of the U.S. studies. The Australian 2D:4D ratios resembled those in Britain rather than those in the U.S., tending to exclude hypotheses related to latitude and making differences in gene pools a plausible explanation.

Psychological masculinity-femininity via the gender diagnosticity approach: heritability and consistency across ages and populations.

Several aspects of the Gender Diagnosticity (GD) approach of Lippa (1995) to measuring the psychological trait of masculinity-femininity within sexes were explored in four samples ranging from 363 to 5,859 individuals, including Swedish and Australian adults, U.S. elderly, and Australian adolescents. Two ways of deriving GD scales yielded highly similar results. Moderate stability of individual differences was found across ages 12 to 16 among adolescents, but substantial shifts over age occurred in relationships with Eysenck scales. Considerable generality of GD scales was obtained across languages and populations. Substantial heritabilities (about 40%) and minimal effects of shared family environments suggest that within-sex masculinity-femininity behaves as a fairly typical personality trait. Cross-age continuity appeared mainly to reflect the influence of the genes.

Genetic and environmental components of adolescent adjustment and parental behavior: a multivariate analysis.

Adolescent adjustment measures may be related to each other and to the social environment in various ways. Are these relationships similar in genetic and environmental sources of covariation, or different? A multivariate behavior-genetic analysis was made of 6 adjustment and 3 treatment composites from the study Nonshared Environment in Adolescent Development, using 674 same-sex adolescent sibling pairs aged 9-11. Cholesky decompositions of the total covariance matrix yielded additive and nonadditive genetic, and shared and nonshared environmental matrices. Factor analyses led to 3 factors for all but shared environment. The first 2 factors resembled Neuroticism and Extraversion factors typically found for personality; the third factor, parental monitoring and control, appeared to have different associations in different matrices.

A reanalysis of five studies on sexual orientation and the relative length of the 2nd and 4th fingers (the 2D:4D ratio).

Five studies have examined the relationship between sexual orientation and the relative lengths of the 2nd and 4th fingers (the 2D:4D ratio). Although differences have commonly been found between heterosexuals and homosexuals, the direction of the difference has not been consistent across studies. The original data from all five studies were reanalyzed in a search for possible explanations of the discrepancies. Because ethnicity is known to affect the 2D:4D ratio, the reanalysis focused on participants who identified themselves as White or Caucasian, the ethnic group that was most numerous in all of the studies. Age differences did not account for the discrepancies. Differences in variability within different groups were minor. One interesting result to emerge from the reanalysis was that the 2D:4D ratios for the homosexual groups were relatively similar across studies. It was the 2D:4D values for the heterosexual participants that varied most, particularly between the USA and the British studies, and these were responsible for many of the discrepancies in the conclusions across studies. The constancy of the 2D:4D ratio for the White homosexuals did not appear to extend to homosexuals of three other ethnicities, and there were also subpopulation differences related to right or left hands.

Gender diagnosticity and androgen receptor gene CAG repeat sequence.

The gender diagnosticity (GD) approach of Lippa (1995) was used to evaluate the relationship of within-sex differences in psychological masculinity-femininity to a genetic characteristic, the length of a repeated CAG sequence in the X-linked androgen receptor (AR) gene. Previously assessed adult samples in Australia and Sweden were used for this purpose. A weak relationship (correlations in the range .11 to .14) was obtained in both countries. Additional data from adolescent twins from Australia (12-, 14-, 16-year-olds) did not confirm such a relationship at those ages, especially for males. The fact that this sample consisted of twins permitted two kinds of within-pair comparisons: (1) Did the dizygotic twin who had the longer AR sequence have the higher GD score? (2) Was one twin's GD score more highly correlated with the other twin's AR score in MZ than in DZ pairs? The answer in both cases was negative. Clarification of these relationships will require large samples and measurements at additional ages.