One reason why this may be the case is increasing consensus that diagnosis of a clinical syndrome constitutes a low quality phenotype. This is because a diagnostic phenotype by its nature is inherently heterogeneous. Two individuals could receive an identical diagnosis with many different permutations of symptoms that entirely do not overlap. Severity introduces further variability, with some individuals barely meeting criteria and others exhibiting the most severe manifestation of the disorder. Moreover, subtypes within disorders have been empirically identified (e.g., Hesselbrock & Hesselbrock, 2006; Johnson, van den Bree, & Pickens, 1996; Moss, Chen, & Yi, 2007), reflecting clusters of characteristics that meaningfully aggregate and may be diversely genetically influenced. Furthermore, the course of substance use disorders is highly variable; large portions of individuals recover with or without formal treatment, while others exhibit stable patterns over long periods of time and still others exhibit a progressive course (e.g., Guttmannova et al., 2011). This means that individuals may be classed together as reflecting the phenotype of interest at one point but actually reflect very different profiles when considered longitudinally. Taken together,
