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.

Fig. 1

Full assortment models for a DZ twin pair with parents, spouses, and offspring: social homogamy (upper panel) and positive phenotypic assortment (lower panel). A additive genetic effects, D genetic dominance, E non-shared environmental effects, C shared environmental effects, f cultural transmission path, w gene-environment correlation, q variance additive genetic effects, x variance shared environmental effects, ρ _z assortative mating co-path, P parent, T DZ twin, Sp spouse, O offspring. Please note that additional siblings (and their spouses and offspring) are not included in the figure for reasons of convenience

Fig. 2

Mean correlation (95% CI) of general intelligence between relatives grouped by degree of theoretical additive genetic similarity and dominance genetic similarity. Observed observed correlation, PA expected genetic similarity (A + D) under phenotypic assortment, No PA, expected genetic similarity (A + D) assuming no phenotypic assortment; 95% CI = 95% confidence interval; correlations are constrained to be equal across twins and regular siblings and across sex; MZ twin–twin MZ, DZ twin–twin DZ/sibling, PO parent-offspring, AVMZ cousins avuncular through MZ, AVDZ cousins avuncular through DZ/sibling; COMZ niece/nephews through MZ, CODZ niece/nephews through DZ/sibling, SP spouse pairs; SMZ sister/brother in law through MZ, SDZ sister/brother in law through DZ/sibling, SMZS spouse–spouse through MZ, SDZS spouse–spouse through DZ/sibling, SAVMZ aunt/uncle cousin in law through MZ, SAVDZ aunt/uncle cousin in law through DZ/sibling, POS parent-offspring in law. Please note that the degree of additive genetic similarity increases within a population undergoing phenotypic assortment for all pairs of relatives except MZ twin pairs. Similar, dominance genetic similarity is induced by phenotypic assortment for AVMZ, AVDZ, COMZ, CODZ, SMZ, SDZ, SMZS, SDZS, SAVMZ, and SAVDZ within a population undergoing phenotypic assortment, where under random assortment there would be none (Fisher ; Nagylaki ; Lynch and Walsh 1998). For the expected correlations we assumed  = .44,  = .27, as estimated under the reduced model and ρ _z = .37 (i.e., the observed spousal correlation); Please see the Appendix for coefficients for σ _A² and σ _D². Under social homogamy, spousal correlations are increased but do not differ as a function of genetic relatedness (i.e., correlations between direct spouse pairs are expected to be the same as correlations between spouses in law). Social homogamy does not affect the genetic relatedness of relatives

Fig. 3

Standardized variance components for general intelligence based on full (left) and reduced (right) SH and PA models. ASM assortative mating (phenotypic assortment under the PA model; social homogamy under the SH model), A additive genetic factors, D genetic dominance, E non-shared environmental factors, C shared environmental factors, CT cultural transmission, rGE correlation between A and CT, Under the PA model, rGE is negative since the unstandardized parameter estimate of CT is negative; model numbering corresponds to model numbering in Table 2