More complete summaries of the findings from candidate gene studies have been published previously (Gelernter & Kranzler, 2009; Kreek, Bart, Lilly, Laforge, & Nielsen, 2005) so here we intend only to make a few points based on those studies we have discussed. As can be seen in Table 1, it is apparent that even for individual genes that are particularly well studied (such as DRD2 and OPRM1) less than half of the studies in the literature identify positive associations. Given that there is well-known bias towards publishing positive results, and against publishing negative results, the actual percentage of positive findings is probably substantially less. This might indicate several possibilities. The positive findings may be false positives and entirely spurious. Given the importance of these genes in the actions of drugs of abuse this appears unlikely. Other solutions, however, present themselves, including the possibility that the effects of any particular genetic variant may be rather small, that the variations may be heterogeneously distributed, and that they may be more specifically involved in particular endophenotypes or addictions associated with particular drugs of abuse. These possibilities will be reconsidered below in the context of the findings from GWAS.
