The implications of these findings are that friends' alcohol use should not be considered an entirely environmental risk factor for adolescent alcohol use and problem use. Genetic influences can be expected to play a role in individual differences in alcohol use by adolescents as well as contribute to who their friends are and to what extent these friends use alcohol. These findings point to the possibility of gene–environment correlations in adolescent alcohol use. However, further research is required to increase understanding of the type and form such correlations take and the processes by which they exert their effects. The high common environment correlation also implies that environmental influences on twins that make them more similar in drinking behaviour are the same as those that make their friends' drinking behaviour more similar to their own. These findings contribute to understanding of the mechanisms by which friends' alcohol use influences adolescent drinking behaviour.
