The amount of heritable variance in each measure was estimated using standard biometric approaches to modeling twin-family data (Neale, Boker, Xie, & Maes, 2003). All measures were adjusted for the effects of covariates (chronological age, gender, generation cohort, recording system, and the 10 EIGENSTRAT PCs), and biometric models fit to the residuals. In typical biometric models three latent variables account for the variance in each phenotype: additive genes (A), common or shared environment (C), and unique or unshared environment (E). The correlations among family members with respect to the three latent variables determine the within-family phenotypic correlations. For instance, the family environment is shared equally by all family members, so the correlation for C is 1, whereas DZ twins and parents and offspring share half their genes, so the genetic correlation is 0.5 in these pairs but 1 in MZ twins. E is by definition unique to each individual. Models were fit to data from four-member families as well as based only on twins, the latter to facilitate comparisons between the present results and published data, which is predominantly based
