However, the comparable design in humans, the adoption design, is problematic for several reasons. Human adoptions tend to select for strong genetic effects (due to deviant biological parents) and low variance in rearing conditions (i.e., adoption agencies tend not to place adoptees in problem homes) (Heath et al, 1997). Thus, it is not necessarily surprising that most human adoption studies have failed to find main environmental effects of exposure to parental alcoholism (Cadoret et al.., 1985, 1987; Cloninger et al., 1985; Goodwin et al., 1974; McGue, Sharma, & Benson, 1996); and such GXE effects as have been identified (e.g. involving adoptive parent marital status: Cadoret et al. 1986) will be of uncertain generalizability to the general population.
