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

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 1918; Nagylaki 1978; 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

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