© 2018 by The University of Chicago. The relationships between mating decisions and parental investment are central to evolution, but to date few theoretical treatments of their coevolution have been developed. Here we adopt a demographically explicit, adaptive dynamics approach to analyze the coevolution of female mating decisions and parental investment of both sexes in a self-consistent way. Our models predict that where females cannot interfere with one another’s mating decisions and where they do not differ in their survival and fecundity prospects, monogamy should be rare, favored only under harsh environmental conditions, in sparse populations. However, allowing for interference or asymmetries among females leads to selection for monogamy over a much broader range of environments and demographies. Interference by paired, resident females may prevent unmated rivals from joining existing monogamous pairs, thus barring the formation of polygynous groups. Asymmetries between established, primary females and subsequently joining secondary females may increase the relative costs of early polygynous reproduction, compared to delayed monogamy for the latter. The models thus highlight different routes by which monogamy may evolve. We further track how parental investment by the sexes coevolves with female mating decisions, highlighting how sexual conflict over parental investment is both cause and effect of mating behavior.