In view of the fact that the heritability of AD is in the range of 50–60%, there is considerable environmental influence on the phenotype, as is the case for other complex traits. However, unlike many other such traits, environmental factors are a necessary component of AD. One cannot become substance dependent without exposure to the substance, regardless of one’s genetic constitution. This places a trait like AD in contrast to a trait like schizophrenia, which, so far as is known, does not require any special environmental exposure to develop. It has been appreciated only recently that G × E effects can be detected reliably for psychiatric traits and specific candidate loci using samples of a size that can feasibly be recruited (see Caspi and Moffitt 2006 for a review), whereas the impact of environmental factors on genetic risk of AD related to alcohol metabolizing enzymes has a longer history (e.g., Heath et al. 2001).
