Using data from the Fragile Families and Child Well-Being Study, the present study tested whether the benefits of a marital birth for early child development diminish as parents' risk of having a nonmarital birth increases (N = 2,285). It was hypothesized that a child's likelihood of being born to unmarried parents is partly a function of father characteristics that predict his capacity to promote child development. Results partially supported hypothesis. A positive association emerged between parental marriage and cognitive outcomes at age 3 only for children whose parents were likely to be married at the child's birth, suggesting average differences between children in married and unmarried families may overestimate the benefit of marriage in subpopulations most impacted by nonmarital birth.