Marriage regimes exist in many guises and forms. Economists have studied monogamy and polygyny, the two most commonly encountered types, and pointed to various benefits that can explain why and which individuals form conjugal unions in each regime. However, many of these same benefits should favor polygamy over monogamy more generally, including polyandrous and cenogamous marriages, which are only rarely observed in practice. We show that human reproductive technology in combination with regime-specific potential for conflict among parents of the same and opposite sex over resources devoted to own children can explain why monogamy is most common, polygyny frequent, polyandry rare, and cenogamy virtually non-existent. Within-wives conflicts over resources also provide an alternative explanation for why polygyny has historically been less common than monogamy and why the former has declined in many parts of the world over the last century.