Advances in genetic research of SUDs depend heavily on optimal phenotypic measurement. For SUDs, this is a particularly challenging issue. It is not clear that the traditional categorical models used in psychiatry are optimal for SUDs1. It may make little sense to apply—as done in DSM-IV—the same set of criteria for dependence to all substances of abuse. These criteria were selected using expert consensus, but there is no a priori reason to assume that they are optimal or that SUDs are distinct categories in nature rather than continua. Fortunately, however, these issues are open to empirical investigation, and there has been progress, both methodological and substantive, in this area in recent years30,31.
