Biological factors in addictions operate at two levels. First, there are factors that increase the likelihood that some people will develop a substance addiction. Second, once an addiction has been acquired, other biological mechanisms cause the addiction to be maintained. The strongest evidence for a biological role in addictions comes from the field of genetic epidemiology. Numerous long-term studies of people who were adopted at an early age have shown that the risk for developing certain types of alcoholism is four to nine times greater in those whose natural parents were alcoholics than it is in those whose natural parents were not.
Biochemical research provides other evidence for genetic predisposition to addiction.
It has become increasingly clear that the primary biological pathways for developing alcoholism and other drug addictions are alterations of normal biochemical processes involving metabolism of these substances in the liver and the effects these drugs have on various neurochemical systems in the brain. For example, the key factor in genetic predisposition to alcoholism appears to be the presence of alcohol dehydrogenase II, a previously unknown liver enzyme. Those who have this enzyme can metabolize alcohol much more efficiently, are able to drink large amounts of alcohol without becoming intoxicated, and are therefore more vulnerable to becoming addicted to alcohol. Not all addictions have a genetic basis. A host of behavioral, social and cultural factors appears to play a part in the development of some forms of substance addiction.