Twin studies use the fact that monozygotic (MZ) pairs share 100% of their genetic variation, whereas dizygotic (DZ) twins share on average 50% of their genetic variation, to partition sources of variation in a phenotype into main effects from three latent components: additive genetic (A) variance shared by twins due to genetic alleles that combine additively, common environmental (C) variance that comes from experiences that make members of a twin pair similar to each other, and individual-specific (non-shared) environmental (E) variance that makes members of a twin pair different from each other. Estimation of each source’s (i.e., A, C, E) contribution to individual differences in a phenotype is made by comparing the similarity of samples of MZ and DZ twin pairs3. In a traditional twin design, interactions between genetic and common environmental factors are confounded with estimates of genetic main effects, and interactions between genetic and individual-specific environmental factors are confounded with estimates of individual-specific environmental effects (Eaves, Last, Martin, & Jinks, 1977; Heath & Nelson, 2002). If the twin model is expanded by including explicit measures of the environment
