Alcoholism runs in families. This is likely a result of the transmission of genetics as well as the familial environment. Twin methodology has been used to study the measurable contribution of genetic and environmental influences on a particular trait or disease state. Twin studies provide an estimation of a trait’s heritability in a population; that is, what proportion of phenotypic variation is produced by genetic variation underlying the trait. Twin studies accomplish this by comparing phenotypic similarity between monozygotic twins, who share all of their genetic variation, with dizygotic twins, who share (on average) half of their genetic variation. Measures of heritability are a function of the specific population. Heritability of substance use disorders varies among substances (and the measure of substance use), populations, age, and sex. A 2006 meta-analysis of twin studies showed that the heritability of all addictive substances ranges from 40% to 60%.27 Heritabilities in the range of 30% to 60% are also observed for illicit drug dependences.28,29 Variability in the exact estimates is likely a function of the age of the participants (genetic influences often increase
