Genetic analyses conducted with twins reared together are used to decompose the variance of a phenotype into additive genetic sources (A), shared environmental sources (C; influences that make members of a twin pair more alike, such as family and school) and environmental influences that contribute to within-pair differences (E). Genetic influence is suggested by higher correlations in MZ than DZ twin pairs for the phenotype ; shared environmental influence is indicated by DZ twin correlations exceeding half of MZ twin correlations. Non-additive sources of genetic influence (D) can be estimated in place of C when the correlation between members of DZ twin pairs is less than half of that between their MZ counterparts. However, as C and D cannot be estimated jointly when data from twins alone are used, the choice of an ACE versus an ADE model is made based on results of twin correlations. In our sample, the DZ correlations were greater than half the MZ correlations for trauma exposure (rDZ=0.51, rMZ= 0.60) and AD (rDZ=0.45, rMZ=0.71). In the case of PTSD, the twin correlation for DZs was
