To examine whether the increased rate of habitual smoking among the siblings of alcoholic probands was mediated through an increased rate of alcohol dependence among those siblings themselves, the investigators examined the smoking rates of alcohol-dependent and non-alcohol-dependent siblings of alcoholics. If the increased rate of habitual smoking seen in siblings of alcoholics was mediated only through an increased rate of alcohol dependence, one would expect the rate of habitual smoking in the non-alcohol-dependent siblings to be the same as in the comparison sample. The study found, however, that compared with the community-based sample, an increased risk of habitual smoking existed even among non-alcohol-dependent siblings (see table 2), suggesting that factors in addition to an increased rate of alcohol dependence accounted for the siblings’ risk of being habitual smokers. Indeed, part of this increased risk depended on the alcoholic proband also being a habitual smoker—that is, siblings of smoking alcoholic probands were at higher risks of smoking themselves than were siblings of nonsmoking alcoholic probands, indicating in part a specific familial transmission of habitual smoking. Siblings share many characteristics that
