Quantitative Genetics, Molecular Genetics, and Intelligence

Genetics and high cognitive ability

More is known about the genetics of general cognitive ability (g) than any other trait in psychology. Recent findings on the genetics of g include the following three examples: (1) heritability increases throughout the lifespan; (2) heritabilities of performance in cognitive tests are strongly correlated with the tests' loadings on a g factor; and (3) genetic effects on scholastic achievement largely overlap with genetic effects on cognitive ability. This body of genetic research addresses the aetiology of individual differences in the normal range. Much less is known about the genetics of the high end of the distribution. Finding heritability in the normal range of cognitive ability does not imply that high ability is also genetic in origin. However, the first twin study of high IQ children, which uses a new technique that analyses the average difference between extreme groups and the rest of the population, suggests that high IQ is as heritable as individual differences in the normal range. We are currently engaged in a molecular genetic study that attempts to identify specific genes that contribute to high ability.

The biosocial female choice theory of social stratification

For decades, the study of social stratification has been dominated by environmental theories. Herein a theory is proposed that contains both biological and sociocultural elements. The theory asserts that most human females, like females of many other mammalian species, have evolved mating preferences biased toward males who are competent in provisioning resources. This female bias is hypothesized to have been naturally selected because females with these biases nearly always have had a reproductive edge over females who lack such a bias. One result of this bias is that human females preferentially mate with males who strive to rise in social status. This, in turn, has favored males who attain or at least strive for high social status, and who advertise and even exaggerate whatever status they already have achieved. At the genetic level, the theory postulates that alleles have accumulated on the human genome that promote social status-striving and achievement to varying degrees. To account for why males are more prone toward status-striving than females, the theory contends that one or more genes on the Y-chromosome interact with genes on the remaining human chromosomes to incline males to gravitate toward social hierarchies and to strive for niches that are relatively high in those hierarchies. Both tested and untested hypotheses are derived from the theory and compared to the empirical evidence currently available.

Mental retardation: genetic findings, clinical implications and research agenda

The most important genetic advances in the field of mental retardation include the discovery of the novel genetic mechanism responsible for the Fragile X syndrome, and the imprinting involved in the Prader-Willi and Angelman syndromes, but there have also been advances in our understanding of the pathogenesis of Down syndrome and phenylketonuria. Genetic defects (both single gene Mendelizing disorders and cytogenetic abnormalities) are involved in a substantial proportion of cases of mild as well as severe mental retardation, indicating that the previous equating of severe mental retardation with pathology, and of mild retardation with normal variation, is a misleading over-simplication. Within the group in which no pathological cause can be detected, behaviour genetic studies indicate that genetic influences are important, but that their interplay with environmental factors, which are also important, is at present poorly understood. Research into the joint action of genetic and environmental influences in this group will be an important research area in the future.

Genetic and environmental influences on perceptions of self-worth and competence in adolescence: a study of twins, full siblings, and step-siblings

Although it is generally assumed that the origins of adolescents' perceptions of self-competence lie in shared family environmental influences, the contributions of nonshared environmental or genetic influences have not been explored. We investigated sibling resemblance for perceived competence and self-worth in 720 adolescent pairs aged 10 to 18 years, using a twin, full sibling, and step-sibling design. Our goals were to assess the magnitude of shared and nonshared environmental influences and to disentangle resemblance due to shared genetic heritage from that due to shared environmental experiences. Shared environment was not significant for any of the scales. 4 of the subscales showed significant genetic influence: scholastic, social, physical, and athletic competence. We also explored possible sources of genetic influences on perceived competence. Bivariate models revealed common genetic variance between scholastic competence and vocabulary and social competence and sociability. These measures, however, did not account for all of the genetic variance in perceived social and scholastic competence.

Variability and stability in cognitive abilities are largely genetic later in life

The powerful quantitative genetic design of identical and fraternal twins reared apart (112 pairs) and matched twins reared together (111 pairs) was employed to assess the extent of genetic influence on individual differences in cognitive abilities during the last half of the life span. General cognitive ability yielded a heritability estimate of about .80 in two assessments 3 years apart as part of the Swedish Adoption/Twin Study of Aging. This is one of the highest heritabilities reported for a behavioral trait. Across the two ages, average heritabilities are about .60 for verbal tests, .50 for spatial and speed-of-processing tests, and .40 for memory tests. For general cognitive ability, the phenotypic stability across the 3 years is .92 and stable genetic factors account for nearly 90% this stability. These findings suggest that general cognitive ability is a reasonable target for research that aims to identify specific genes for complex traits.

DNA markers associated with high versus low IQ: the IQ Quantitative Trait Loci (QTL) Project

General cognitive ability (intelligence, often indexed by IQ scores) is one of the most highly heritable behavioral dimensions. In an attempt to identify some of the many genes (quantitative trait loci; QTL) responsible for the substantial heritability of this quantitative trait, the IQ QTL Project uses an allelic association strategy. Allelic frequencies are compared for the high and low extremes of the IQ dimension using DNA markers in or near genes that are likely to be relevant to neural functioning. Permanent cell lines have been established for low-IQ (mean IQ = 82; N = 18), middle-IQ (mean IQ = 105; N = 21), and high-IQ (mean IQ = 130; N = 24) groups and for a replication sample consisting of even more extreme low-IQ (mean IQ = 59; N = 17) and high-IQ (mean IQ = 142; N = 27) groups. Subjects are Caucasian children tested from 6 to 12 years of age. This first report of the IQ QTL Project presents allelic association results for 46 two-allele markers and for 26 comparisons for 14 multiple-allele markers. Two markers yielded significant (p < .01) allelic frequency differences between the high- and the low-IQ groups in the combined sample-a new HLA marker for a gene unique to the human species and a new brain-expressed triplet repeat marker (CTGB33). The prospects for harnessing the power of molecular genetic techniques to identify QTL for quantitative dimensions of human behavior are discussed.

Differences in heritability across groups differing in ability, revisited

Three recent studies have used twin data to explore the possibility of differential contributions of heritability and environmentality to individual differences in cognitive ability as a function of ability level (Detterman, D. K., et al., Behav. Genet. 20:369-384; 1990; Bailey, M. J. and Revelle, W., Behav. Genet. 21:397-404, 1991; Cherny, S. S., et al., Behav. Genet. 22:153-162, 1992). All arrived at different conclusions: higher heritability at the low end, higher heritability at the high end, and no differential influence, respectively. The current report involves a sample of 148 identical and 135 fraternal twin pairs from the Western Twin Project who were tested on a battery of intelligence and achievement tests to further explore the issue. The results suggest no significant differences in heritability at either the high or the low end, although a trend toward higher heritability for children of higher ability is evident. Individual differences for a composite ability/achievement score showed significantly greater influence of shared family environment at the low end than the rest of the distribution. In general, results for cognitive ability and academic achievement were highly similar.

Quantitative trait loci and psychopharmacology: response to commentaries

The theme of our article was that a merger is needed between quantitative genetic and molecular genetic approaches in order to detect genes associated with psychopharmacological processes even when the genes account for small amounts of variance, so-called quantitative trait loci (QTL). The recombinant inbred (RI) QTL approach using the BXD RI series was discussed as a promising approach.The commentaries by Crabbe and by McGuffin and Buckland make several excellent points to which we have little to add. Goldman and Katz, on the other hand, disagree with some of our arguments and for this reason much of the limited space of our response to the commentaries is directed towards the commentary by Goldman and Katz. We focus on three general issues that they raise: the relationship between quantitative genetics and molecular genetics (single genes vs multiple genes), reverse genetics (anonymous markers) vs forward genetics (candidate genes) and heterogeneity (narrow vs broad assessment). Our response to Goldman and Katz is that these are false dichotomies-we do not need to choose sides between major- and multiple- gene approaches, reverse and forward genetics, or narrow and broad assessment. Rather than choosing sides, we should encourage the deployment of multiple research strategies in order to maximize the probability of identifying genes that affect behavior. A major strength of the RI QTL approach is that it can identify both major- and multiple-gene effects, it employs both reverse and forward genetics, and it can be applied to both narrow and broad assessment (and its multivariate extension is ideally suited to understanding the genetic interrelationships among different levels of assessment).After discussing these general issues raised by Goldman and Katz, we address two specific issues raised in the commentaries: multivariate analysis of multiple markers and the use of F(1) crosses between RI strains to increase power. We end by mentioning the establishment of an RI QTL collaborative registry which aims to facilitate RI QTL analyses.