Sixth, prior twin studies of externalizing disorders did not test for qualitative sex effects—that is the degree to which the genetic and shared environmental influences on these syndromes were the same in men and women. We examined this question and were surprised to find that the best fit model constrained these correlations to equality both for the externalizing common factor and for the syndrome specific influences. Consistent with the one relevant power analysis (Prescott and Gottesman 1993), further analyses suggested, that despite our large sample size, we actually had rather limited power to discriminate different models. Other models that fit slightly less well suggested that the correlation between sexes might be slightly less than unity but to be high and similar for genetic and for shared environmental influences. They also suggested that the sharing of genetic influences might be modestly lower for CB than for DA or AUD. But we should regard these findings as only suggestive and worthy of follow-up.
