Which sex should care for offspring depends on the cost and benefits of the behaviour for each sex. Understanding these differences between the sexes is a fundamental step to explain the evolution of animal societies, but it is often difficult to quantify them empirically. A possible approach is to investigate two closely related species that perform a very similar type of care but in which the caring sex differs. 2. Using field and laboratory data, we estimated the benefits and costs of parental care in two species of assassin bugs with very similar ecologies: Rhinocoris tristis, which has exclusive paternal care, and Rhinocoris carmelita, which has exclusive maternal care. 3. In both species, the main benefit of care was a reduction in parasitism and predation of eggs. Guarding R. tristis males consumed eggs (filial cannibalism), and thus managed not to lose weight, but R. carmelita females paid the full energetic cost of care. Guarding male R. tristis incurred survival costs relative to non-guarding male and female conspecifics. 4. Very high population density and female preference for males already guarding eggs (a preference previously recorded in fish) minimised the promiscuity cost of paternal care in R. tristis, explaining the difference in care pattern between the two species.