For regular drinking we observed a similar pattern as for
frequency of drinking. In male twins concordant for
friends’ alcohol use, the variance in the liability to regular
drinking was explained for 4% by age, and for 14% by
additive genetic effects, 66% by common environmental
effects and for 16% by unique environmental effects. We
found a similar pattern for female twins, with estimations
for the effects of age, genes, common environment and
unique environment at 0, 15, 69, and 16%, respectively.
When twins were discordant for the alcohol use of their
friends, the variance in males was explained for 7% by age
and for 33% by additive genetic effects, while common