The etiology of OD is complex and multifactorial with a substantial heritable component (Gelernter et al., 2006; Kendler et al., 2003; Saxon et al., 2005). However, the heterogeneous phenotype defined by the DSM-IV diagnosis of OD (American Psychiatric Association, 1994) does not lend itself readily to gene finding (Gelernter et al., 2006). Thus, the identification of valid and homogeneous subgroups based on opioid use and related behaviors can refine the phenotype and enhance genetic analysis. This subtyping approach could facilitate the development of new treatments targeted to specific subgroups, enhancing personalized care (Chan et al., 2011; Gelernter et al., 2006; Kranzler et al., 2008; Sabb et al., 2009). In the present study, we sought to identify opioid use subgroups that 1) differed significantly on clinical features and 2) demonstrated high heritability. Heritability is the ratio of additive genetic variance to the total phenotypic variance within a population (Bell 1977). Although the heritability of a trait reflects the genetic contribution to that trait in the population, it cannot be directly applied to estimate the likelihood that an individual in that population will have the trait.
