Within both the U.S. and Scandinavian studies, no significant gender differences were found in the genetic contribution to alcoholism risk. In the Scandinavian data, genetic factors appear to be more important in women than in men (a pattern that is seen in both the Swedish adoption and Swedish twin studies), but no statistically significant difference exists. Based on the U.S. data, genetic effects account for approximately 60 percent of the variance in alcoholism risk in both men and women, and the twin data suggest that there is no significant effect of family environment. The U.S. adoption data suggest that the adoptees’ family environments may account for one-third of the variance. The Scandinavian data yield a lower estimate for the importance of genetic influences (i.e., 39 percent) and a modest but significant estimate for family environmental influences (i.e., 15 percent). This difference between the U.S. and Scandinavian data appears to be explained largely by differences in Scandinavian males, with estimates for Scandinavian women being close to those for U.S. men and women.
