We present a game theoretic model to explain why people form life long monogamous fam- ilies. Three components are essential in our framework, paternal investment, fatherhood uncertainty, and, perhaps the most distinctive feature of all, the overlap of children of dif- ferent ages. When all three conditions are present, monogamy is the most e cient form of sexual organization in the sense that it yields greater survivorship than serial monogamy, group marriage, and polygyny. Monogamy is also the only con guration that fosters al- truistic ties among siblings. Finally, our result sheds light to the understanding of why most religions center around the monogamous delity family.