Evidence for significant heritability for all of the addictive disorders has led to considerable efforts to identify the specific genes involved. The gene search has been complicated by addictive behaviors being complex genetic traits that are phenotypically and genetically heterogeneous. It is expected that there are multiple genetic loci influencing manifestation and variation in these behaviors, and that these loci vary in the direction and magnitude of their effects. Beyond the multiple loci involved, it is likely that there are also many layers of intricate interactions between loci (epistasis), in addition to gene-environment interactions.
