The aim of the project is to investigate the genetical and environmental causes of individual differences in drinking habits and alcohol consumption. Evidence exists from studies in other countries that genetic factors play a part in this variation. There is also evidence that cultural transmission and the similarity of husbands and wives are important familial influences on drinking habits. No study has yet investigated all these potentially important causes of variation. Questionnaires on drinking habits and alcohol consumption will be mailed to 3,600 pairs of monozygotic (MZ) and dizygotic (DZ) twins aged 21-40 and ascertained through the Virginia Twin Registry. Replies from 2,500 complete pairs are anticipated and similar questionnaires will be mailed to their parents, sibs, spouses and parents-in-law. Replies from 7,500 non-twin relatives are expected, yielding a total sample size of 12,500 individuals. Complex path models will be fitted by maximum likelihood methods to these data to test the importance of the following potential influences on variation in drinking habits: inherited factors; cultural effects of a) parents on their children; b) children on their parents; c) siblings on each other; d) environmental factors outside the family; assortative mating. These effects may differ between sexes and vary with age. To test whether marital correlation for alcohol consumption is due to assortative mating or convergence in drinking habits following marriage, a sub-sample of 250 pairs of newlywed spouses will be sent a second questionnaire 18 months after the first. A subsidiary aim is to ascertain, through appropriate items on alcohol abuse and dependence, a sample of twins who are potential alcoholics and who will form the basis of a subsequent, independent project on genetic and environmental influences on alcoholism.