Table 1.

Explanation of variance components in ETFD models.

Parameter	Interpretation
VP, σ2	Phenotypic variance.
VP̃	Variance of latent phenotype upon which mates choose each other.
VA	Additive genetic variance; variance of marginal or average allelic effects.
VD	Dominance genetic variance; variance of effects attributable to combinations of alleles at the same locus.
VS	Sibling environmental variance; variance in nongenetic effects (e.g., peers, cohort, school, parenting style, and so on) shared between siblings and twins but not between parents and offspring.
VT	Twin environmental variance; variance in nongenetic effects (e.g., peers, cohort, classrooms) shared by twins but not siblings.
VF	Familial environmental variance; variance in nongenetic effects (e.g., SES, social mores, education) passed (via ‘vertical transmission’) from parents to offspring.
VC	VS + VF; typically estimated in CTD or NTFD models.
VE	Unique or residual environmental variance; variance in nongenetic effects (e.g., peers, unique experiences, somatic mutation) that are unshared with any other relative class.
CV(A, F)	Covariance between additive genetic and familial environmental effects; arises if vertical transmission (causing VF) is a function of the parental phenotype because, for example, higher values on A create higher phenotypic values, which are passed to offspring F via vertical transmission.