The Rule of One-Third guaranteed wives a life interest in one-third of their husband's estate upon marital dissolution. We document the ubiquity of this legal construct over four thousand years and across numerous societies. Due to specialization within the household, we demonstrate that without a wife's residual claim on her husband's estate, children's outcomes are imperiled. Using ancient Roman law as an example, we argue that the patriarch, or paterfamilias, is the primary legal entity with an interest in creating and enforcing the Rule of One-Third. Then, in a game-theoretic model, we demonstrate that the Rule of One-Third obtains when mothers' and fathers' are equally important at producing children's human capital, and when this rule is enforced by the paterfamilias or by modern legal institutions. We conclude that the Rule of One-Third arose in many societies because it places the cost of marital dissolution on the household rather than society, and solves a contracting problem between the husband and wife when each is specialized in tasks the other cannot perform well.