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 large majority of human behavioral traits (44). If the association between marital status and alcohol use disorder resulted from such confounding factors, we would expect that it would be modestly weaker in distant relationships and then become further weakened among more closely related relative pairs who share more of their genetic and environmental background. In particular, the expectation would be that among informative monozygotic twin pairs, who share all of their genetic and rearing environment, the association would be substantially attenuated compared with that observed in the general population. This is not the trend observed in our data. Rather, we see no weakening of the association between marital status and alcohol use disorder as we move from the general population to cousins, half siblings, full siblings, and, finally, monozygotic twins discordant for marital status. These results provide support for the second hypothesis—that marriage is causally related to a reduction in risk for alcohol use disorder. This conclusion is consistent with
