Twin studies have provided the leverage needed to disentangle the relative importance of genetic and environmental influences on the generalized liability for substance use problems. Findings from these studies suggest that genetic factors explain a large degree of the overlapping risks among alcohol, tobacco, and marijuana use problems in youth (Hopfer et al., 2003; Kendler et al., 2007, 2008; Young et al., 2006). The search for specific genes that may explain this inherited liability for substance use problems is challenging and ongoing. Meta-analyses across published linkage and association studies point to polymorphisms in genes involved in the metabolism of drugs (e.g., polymorphisms in the alcohol dehydrogenase gene; Dick and Beirut, 2006; Li et al., 2008) and polymorphisms involved in neurological systems (e.g., brain-reward; Dick and Beirut, 2006; Gardner, 2002; Koob and Le Moal, 2001) that are widely known to be involved in the addiction process. Despite this progress, efforts to identify specific genetic polymorphisms that account for the generalized biological risk underlying substance use and SUDs continue.
