A parallel effect was found among the adult twins: Genetic effects accounted for less than 60% of the variance in married respondents but more than 76% in unmarried respondents (Heath et al. 1989). In an independent sample of Dutch twins, religiosity was also shown to moderate genetic and environmental influences on alcohol use initiation in females (with nonsignificant trends in the same direction for males): In females without a religious upbringing, genetic influences accounted for 40% of the variance in alcohol use initiation compared to 0% in religiously raised females. Shared environmental influences were far more important in the religious females (Koopmans et al. 1999). In data from our population-based Finnish twin sample, we also found that regional residency moderates the impact of genetic and environmental influences on alcohol use. Genetic effects played a larger role in longitudinal drinking patterns from late adolescence to early adulthood among individuals residing in urban settings, whereas common environmental effects exerted a greater in-fluence across this age range among individuals in rural settings (Rose et al. 2001). When one has pairs discordant for exposure, it is also possible to ask about genetic correlation between traits displayed in different environments.
