Although there have been several studies of twins reared apart,24 because of the rarity of such pairs most research examines twins reared together. These studies compare concordance rates between members of monozygotic/identical and dizygotic/fraternal twins for a trait or disorder, with a higher degree of similarity in MZ (who share 100% of their genes) than in DZ twins (who, on average share 50% of their segregating genes) being interpreted as providing evidence for heritable influences on the behavior being studied. As shown in Figure 1, the correlation between a pair of twins can be expressed as a path diagram, where rectangles represent observed behavior and circles represent latent sources of variation: additive genetic (A), shared environmental (C) and non-shared environmental (E). Heritability is the proportion of total variance in behavior that is attributed to additive (and, in the case of broad heritability, non-additive, when they play a role) genetic factors. Several assumptions and potential limitations underlying the analysis of twin data that need to be considered25, including: a) whether the equal environment assumption (which posits that the correlation for C
