Early examples of gene-environment interaction in twin models necessitated “grouping” environments to fit multiple group models. The basic idea was simple: Fit models to data for people in environment 1 and environment 2 separately and then test whether there were significant differences in the importance of genetic and environmental factors across the groups using basic structural equation modeling techniques. In an early example of gene-environment interaction, data from the Australian twin register were used to test whether the relative importance of genetic effects on alcohol consumption varied as a function of marital status, and in fact they did (Heath et al. 1989). Having a marriage-like relationship reduced the impact of genetic influences on drinking: Among the younger sample of twins, genetic liability accounted for but half as much variance in drinking among married women (31%) as among unmarried women (60%). 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,
