Furthermore, people, at least men, who reported not having histories of alcohol dependence but who were at higher-than-average genetic risk, because they were monozygotic co-twins of alcohol-dependent participants, showed the same diminished responses to alcohol in the challenge study. And nondependent people at intermediate genetic risk who had dizygotic co-twins with histories of alcohol dependence had scores that fell between the two former high-risk groups and the control group (Heath et al. 1999). For reasons not yet understood, comparable associations were not observed in women.
