Having Children with a First Cousin Is Still Not a Good Idea

Consanguinity (parents who are second cousins or closer) occurs in 1 out of 10 births worldwide, and marriage of first cousins is illegal in only three countries: the U.S., North Korea, and China. Even in these countries, relatives still might have children — and children of consanguineous marriages may migrate to the U.S. To examine risk for mental illness in children of cousins versus unrelated parents, researchers used national databases and household interviews in a retrospective, population-wide, cohort study of some 364,000 people born in Northern Ireland over a 15-year period. In all, 609 (0.2%) had parents who were first or second cousins.

In analyses controlling for factors associated with poor mental health, children of first cousi…