of other disorders that may in turn contribute to risk of alcohol dependence. For children of nonalcoholic twins, the genetic risk for alcohol dependence depends on the alcohol dependence history and kind of genetic relationship of their parents’ co-twins. Thus, the genetic risk of children of a nonalcoholic twin will be highest if their parent has an identical twin who is alcohol dependent, intermediate if their parent has a fraternal twin who is alcohol dependent, and lowest if their parent has a fraternal or identical twin who is not alcohol dependent. Furthermore, GxSE interaction effects are expected to contribute to risk most strongly in children reared by an alcohol-dependent parent (that is, children who are at high genetic and high environmental risk), but to have reduced effects in children reared by a nondependent parent whose identical twin is alcohol dependent (i.e., children who are at high genetic risk but lower environmental risk).
