Several study designs, including twin, family, and adoption studies, are used to determine whether relatively common diseases, such as alcohol dependence, are caused at least in part by genetic factors and to estimate the magnitude of the overall genetic contribution. Twin studies compare the similarity in disease status (i.e., concordance2) between identical (i.e., monozygotic) and fraternal (i.e., dizygotic) twins. If risk for a disease (e.g., alcohol dependence) is determined at least in part by genetic factors, monozygotic twins, who have identical genetic material (i.e., genomes), would be expected to have a higher concordance rate for alcohol dependence than dizygotic twins, who on average share only half their genome. Studies by several groups have indeed shown higher concordance rates for alcohol dependence among monozygotic than among dizygotic twins (Agrawal and Lynskey 2008). Family studies, which evaluate the members of a family (both alcoholic and nonalcoholic members) for the presence of the disease, also have provided convincing evidence that the risk for alcohol dependence is determined partly by genetic influences (Gelernter and Kranzler 2009). Overall, family, adoption,3 and twin studies provide convergent
