Table 1:
Definitions of the parameters obtained via twin modeling
| Parameter | Model | Definition |
|---|---|---|
| A | CT NTF | Additive genetic variance: the effect of individual genes summed over loci; acts to increase familial correlations (either between twin siblings or between parents and their biological children) relative to the proportion of genes shared |
| D | CT NTF | Dominant genetic variance: non-additive interactions between alleles at a single genetic locus (e.g., the interaction between dominant and recessive alleles in the determination of eye color); because they involve interactions between alleles, D do not contribute to similarity between parents and their biological children (since each parent provides only one of the two alleles in question); also yield MZ correlations that more than twice as large as those seen for DZ twins |
| C | CT | Shared environmental variance: environmental influences common to family members that act to make them similar to each other regardless of the proportion of genes shared; C=S+F |
| S | NTF | Sibling environmental influences: those shared environmental influences that create similarity between siblings (e.g., exposure to common peers, school, and parenting style), but not between parents and their children |
| F | NTF | Familial environmental variance: shared environmental influences passed via vertical “cultural transmission” between parents and their offspring (e.g., socioeconomic status, social mores); create similarities between siblings and between parents and their children |
| E | CT NTF | Non-shared environmental variance: environmental influences that serve to differentiate family members regardless of the proportion of genes shared; measurement error is also included here |
| w | NTF | Covariance between additive genetic and familial environmental effects: reflects the extent to which the familial environment is a function of the genetically influenced preferences/ tendencies of the parent; also referred to as a passive gene-environment correlation (passive rGE) |
| μ | NTF | Assortative mating copath: spousal similarity on the trait in question; is assumed to be a function of primary phenotypic assortment, whereby mates choose each other based on phenotypic similarity |
Note: See also Keller et al. (2010). Reprinted with permission from Burt & Klump (2012). CT = Classical Twin Model. NTF = Nuclear Twin Family Model.