Genes, environment, and addictive behavior: etiology of individual differences and extreme cases

There is considerable interest in assessing the role of genetic factors in the aetiology of addiction. This paper reviews the possible contribution of a new quantitative technique, termed DF analysis after its originators DeFries & Fulker. DF analysis provides a way of examining whether aetiological factors relating to addiction as a discrete disorder (manifest as extreme scores on a diagnostic instrument) are similar to or different from those that relate to it as a continuum. For example, it is possible to estimate the contribution of familial factors to the development of a clinical condition defined in terms of an extreme score on an instrument by examining the extent of regression towards the unselected population mean of siblings' scores on that instrument. At the same time it is possible to estimate familial influences on the full range of values by examining the correlation between the scores of siblings. Insofar as the two methods produce different estimates, it is possible to infer different levels of involvement of familial factors for extreme cases than for the continuum as a whole. Information provided by such analyses could be critical when constructing models of addictive disorders and help direct more specific studies isolating particular mechanisms.