Drug abuse (DA), a major worldwide public health problem (World Health Organization, 2010), is a highly familial syndrome (Bierut et al., 1998; Merikangas et al., 1998). Therefore, an important research goal has been to understand the nature of these familial risks. Prior twin and adoption studies have consistently shown a substantial etiologic role for genetic factors in the etiology of DA (Kendler et al., 2000, 2012, 2013a; Kendler and Prescott, 1998; Lynskey et al., 2002; Tsuang et al., 1996; van den Bree et al., 1998).
