The biology of some genes might allow for common, functional allelic variants that could escape selective pressures or exert balancing selection over many generations. However, other genes might not be able to harbor such allelic variations without engendering selective pressures that would reduce the frequency of one of the allelic variants in the population over time. Common allelic variants that are able to influence addiction vulnerability are thus likely to be restricted to a subset of the genes whose products are involved in addictive processes. An important consequence of this logic follows: if a gene fails to display variants that influence vulnerability to addiction, the gene’s products are not at all excluded from involvement in addiction. On the other hand, convincing data that implicates a gene’s common variants in addiction should prompt us to consider mechanisms whereby such variants might provide balancing (eg both positive and negative) selective influences in the differing environments through which the ancestors of current human populations have passed.
