In 1960, marriage was a virtual precondition for childbearing. By 1997, out‐of‐wedlock births accounted for 26 percent of fertility among whites and 69 percent among blacks. This paper presents a model that integrates economic theories of fertility and marriage to help understand the growth of out‐of‐wedlock childbearing. In the theory, fathers can shift the costs of child rearing to single mothers. If females are in excess supply and have sufficiently high incomes, a marriage market equilibrium may exist in which children are born within marriage to high‐income parents, whereas in low‐income groups men father children by multiple partners outside of marriage.