There is now growing evidence that the less prevalent allele (A1) of the D2 dopamine receptor (DRD2) gene is strongly associated with severe alcoholism. Similarly, subjects who abuse illegal drugs or who are obese also show a significantly higher prevalence of the A1 DRD2 allele compared to controls. Moreover, cigarette smokers, both past and current, demonstrate significantly higher prevalence of the A1 allele than nonsmokers. In as much as alcohol, cocaine, opiates, nicotine and food are known to increase brain dopamine levels and activate the mesocorticolimbic dopaminergic reward pathways of the brain, it is hypothesized that an inherited deficit of D2 dopamine receptor numbers in brain reward areas of A1 allelic subject predisposes them to substance abuse problems.