Fifth, the most important question in the large literature on the association of marital status and drinking behavior and alcohol use disorder is the degree to which the association is likely causal. In our study, in which we show that marital status predicts subsequent onset of alcohol use disorder, we have two plausible and non–mutually exclusive causal hypotheses: 1) some set of confounding factors act inversely on the probability of marriage and directly on the risk for alcohol use disorder or 2) marriage reduces the risk of disorder. Our initial analyses with covariates provided evidence against the first hypothesis, but it is difficult to be confident that we identified all the relevant confounding variables. Therefore, we proceeded to co-relative analyses in which we compared the association between marital status and risk for first onset alcohol use disorder in the general population and among informative pairs of monozygotic twins, full and half siblings, and first cousins discordant for marital status. The advantage of a co-relative design is that it controls for any confounding risk factors that are themselves familial—which constitutes the
