Second, one must be careful in the interpretation of twin data. The ideal experiment for twin studies would be to compare identical and fraternal twin pairs reared together and apart. This design allows genetic effects, shared environmental effects, and GxSE interaction effects to be estimated jointly, as indicated by the coefficients summarized in table 1 (see Jinks and Fulker 1970; Eaves et al. 1977; Heath et al. 2002). However, separated twin pairs are extremely rare, particularly among younger cohorts. Therefore, twin studies of the etiology of alcoholism have used identical and fraternal twin pairs reared in the same household.
