Reconsidering the Heritability of Intelligence in Adulthood: Taking Assortative Mating and Cultural Transmission into Account

Abstract

Heritability estimates of general intelligence in adulthood generally range from 75 to 85%, with all heritability due to additive genetic influences, while genetic dominance and shared environmental factors are absent, or too small to be detected. These estimates are derived from studies based on the classical twin design and are based on the assumption of random mating. Yet, considerable positive assortative mating has been reported for general intelligence. Unmodeled assortative mating may lead to biased estimates of the relative magnitude of genetic and environmental factors. To investigate the effects of assortative mating on the estimates of the variance components of intelligence, we employed an extended twin-family design. Psychometric IQ data were available for adult monozygotic and dizygotic twins, their siblings, the partners of the twins and siblings, and either the parents or the adult offspring of the twins and siblings (N = 1314). Two underlying processes of assortment were considered: phenotypic assortment and social homogamy. The phenotypic assortment model was slightly preferred over the social homogamy model, suggesting that assortment for intelligence is mostly due to a selection of mates on similarity in intelligence. Under the preferred phenotypic assortment model, the variance of intelligence in adulthood was not only due to non-shared environmental (18%) and additive genetic factors (44%) but also to non-additive genetic factors (27%) and phenotypic assortment (11%).This non-additive nature of genetic influences on intelligence needs to be accommodated in future GWAS studies for intelligence.Electronic supplementary materialThe online version of this article (doi:10.1007/s10519-011-9507-9) contains supplementary material, which is available to authorized users.

Genetic and environmental contributions to cognitive structure in Australian twins: A reappraisal

“…And except for the phenotypic analyses, the samples consist solely of twins. Moving beyond twin studies to broader family designs as in Vinkhuyzen et al (2012) may help to clarify matters: parent-offspring correlations, for example, are affected by assortative mating, but do not involve non-additive genes. Studies of adoptive families may help unconfound genetic and family environment effects.…”

“…If nonadditive genetic effects are involved for a trait, the genetic correlation for DZ twins for that trait will be lowered below 0.5, and shared environmental effects will be underestimated. Such confoundings may affect estimates of shared environmental contributions for cognitive traits (Vinkhuyzen, van der Sluis, Maes, & Posthuma, 2012), since positive assortative mating is usually observed for cognitive measures. Positive assortative mating in the cognitive domain is believed to operate as a single factor (Eaves, Heath, & Martin, 1984), which tends to be more strongly aligned with verbal cognitive measures than with non-verbal ones (Mascie-Taylor, 1989;Watkins & Meredith, 1981;Watson et al, 2004).…”

No Effect of Genome-Wide Copy Number Variation on Measures of Intelligence in a New Zealand Birth Cohort

“…A large body of work comparing monozygotic and dyzygotic twin sets indicates that approximately half of the variation in human intelligence as measured by standard tests is genetic, with estimates ranging between 40 and 80% [1], [2], [3], [4], [5], [6]. Intelligence is also remarkably tractable and stable, with different groups of cognitive tests ranking participants very similarly [7].…”

Future Efforts in Flynn Effect Research: Balancing Reductionism with Holism

“…Finding such a sample of parents is difficult because parents can be members of different cohorts, and cohort IQ is often changing. Also, IQ tends to display assortative mating [16], which can vary in different IQ ranges and over time.…”