Close-kin marriage, by sustaining tightly knit clan-like structures, may impede development. We use 19th and 20th century US state-level bans on cousin marriage to study the causal effect of tight kinship on economic outcomes. We show that these bans did reduce rates of in-marriage, and that affected descendants therefore have higher incomes and more schooling. We examine the following potential mechanisms, drawn from the literature: geographic mobility, female labor force participation, age at marriage, and genetic effects. Our results are most consistent with increased urbanization leading to the increase in income.