Whether genetics contribute to addiction is controversial and is a derivative of the debated topic of whether genes contribute to behavior at all. The genetic basis for states such as schizophrenia and manic-depression is currently a topic of wide investigation. The behaviors that accompany these states and others appear to be influence both by genetic and environmental factors. It is neither nature nor nurture alone, rather nature and nurture that influence behavior. Such is also considered to be the case with addictive behavior, and has been most thoroughly investigated in cases of alcohol and tobacco.

There are several methods used to study the influence of genetics on behavior. They are family studies, adoption studies, twin studies, twins-reared-apart and animal studies. Through these various kinds of genetic studies, various results have implied that genes do play a role in addiction, most specifically to alcohol and tobacco. This being the case, it is likely that all behaviors are influenced to some degree by genetics, including addiction.

Family studies have shown that the incidence of alcoholism is increased in families where one or more members of the family have been diagnosed with alcoholism. Studies have indicated that the progeny of an alcoholic mother or father have two to four times the probability of becoming alcoholic than does the general population. Although little has been done to show that smoking is influenced by genetics, it is a fact that children of smoking parents are more likely to smoke. How much of this is also an environmental issue is not clear.

Adoption studies have also drawn conclusions that alcohol addiction is genetic. Groups of males that had biological but not adoptive parents that were alcoholics were compared to a group of men that had biological and adoptive parents that were not alcoholics. The former group showed an increased incidence for alcoholism four times greater than the latter. A later study (Goodwin et al., 1974) compared the incidence of alcoholism in adopted-away sons of alcoholics with their biological brothers who had been raised by the alcoholic parents. The incidence of alcoholism was identical. The work of Goodwin (1979) also demonstrated that genetic factors are not as important in determining alcoholism in females.

Twin studies and twins-reared-apart studies are the most powerful tools for evaluating genetic affects on behavior. Unfortunately, few of these studies have been used in alcohol related research. The data that has been generated, though, does support the notion that alcohol addiction is influence by genetic factors. In comparing monozygotic (identical) and dizygotic (fraternal) twins, there are different outcomes. Although twins of either category are associated with higher incidences of alcoholism, those of the monozygotic type have an even greater rate.

Animal studies have been used extensively to evaluate the genetic contributions to alcohol and nicotine addiction. Studies by Collins (1979) showed that genetics influence preference for and initial sensitivity to the severity of withdrawal from alcohol. Further investigations have indicated that both metabolism of and central nervous system sensitivities to alcohol are influenced by genetic factors. Similar conclusions have been drawn with nicotine. Mice studies conducted by Marks et al. (1983) showed that these nervous system sensitivities probably involve different brain receptors specific for nicotine. These receptors play a role in the degree of response the individual will get from the drug.

The degree to which genetic factors play a role in addictive behavior is still unclear and remains to be further investigated. The data do suggest that with alcohol, and to a lesser extent nicotine, genetic factors influence addiction considerably. Neither is influenced solely by genetics and environmental conditions appear to be notably important. An area of research that will further define the roles will be to study the affects of environmental factors on genetically susceptible people. The biochemical approach to understanding addiction is another area where genes may contribution.

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