A test of sex differences within the context of interaction models is challenging. In our study, there was little evidence for sex differences, which is consistent with the literature on diagnostic AD and other measures of alcohol-related problems (Pickens et al., 1991; Heath et al., 1997; Prescott et al., 1999). Shared environmental factors, when detected, may induce sex differences. In our study, we were able to constrain shared environmental influences to zero – however, in the models where age at 1st drink was jointly modeled with AD symptoms, evidence for shared environmental influences on AD symptoms were detected in some instances but found to be completely overlapping with those influences that make members of a twin pair similar for age at 1st drink. Importantly, these shared environmental factors were only detected in female twins, which supports prior findings of McGue and colleagues (1992) demonstrating that shared environmental influences on alcohol-related symptomatology may be more prominent in women while genetic influences are more pronounced in men, particularly those with early onset of AD symptoms and with work by Prescott and colleagues
