Co-twin dependence modifies heritability of abstinence and alcohol use: a population-based study of Finnish twins.

The role of co-twin dependence (twins' closeness or reliance on the co-twin) was examined as a moderator of genetic and environmental influences on alcohol use in adolescence and early adulthood in a large longitudinal population-based study of Finnish twins (FinnTwin16). The associations between co-twin dependence and alcohol use were studied first at an individual level in adolescence (n = 3362) and early adulthood (n = 2912). Then, maximum likelihood models were fit to the two waves of data from same-sex twin pairs to assess the differences and changes in genetic and environmental influences on alcohol use (abstinence, drinking frequency, intoxication frequency); N = 1342 pairs in adolescence, and N = 1078 pairs in early adulthood. Overall, no significant associations were found between co-twin dependence and individual alcohol use. However, co-twin dependence importantly modulated genetic effects on drinking habits, especially in adolescence, but also in early adulthood. Co-twin-dependent twins reported greater similarity in their alcohol-related behavior across all alcohol-use measures at both time points, and the role of genes and environments varied according to co-twin dependence. Shared environmental factors explained most of the variation in drinking among co-twin-dependent twins in adolescence and contributed to drinking to intoxication during early adulthood. In contrast, among co-twin-independent twin pairs, genetic variance contributed significantly to all alcohol-use measures at both time-points. An interdependent sibling relationship is an important modifier of drinking habits, and it appears to reduce the impact of inherited liabilities on alcohol-related behavior especially in adolescence.