Although the results of heritability estimates for drug addiction phenotypes are generally around 50%, there is some variability which may relate to differences associated with addiction to specific drugs or to other aspects of addiction criteria used in these studies. In any case, this can only be a starting point, as a demonstration that there are heritable factors that influence individual differences in vulnerability to addiction. The knowledge that a large proportion of abuse liability is genetic does not constitute an understanding of how these genetic factors induce their effects, which genes are involved or how many genes are involved. The first approach that was used in an attempt to identify specific genes that may contribute to addiction liability was the examination of candidate genes, nominated based upon biochemical and other evidence suggesting that those genes should be involved in responses to drugs of abuse or behavioral features of addiction.
