Parents of undergraduates were asked whether they would be more distressed by the sexual or the emotional infidelity of their child’s partner. Comparing the responses of mothers and fathers may offer a rare, “crucial” instance in which evolutionary and socialization perspectives offer different predictions. Socialization theory, broadly conceived, predicts an effect for the gender of the parent: Consistent with their differing gender socialization histories, mothers should be more distressed by the emotional infidelity of their child’s partner, and fathers by the sexual infidelity of their child’s partner. In contrast, an evolutionary psychology approach predicts little or no effect for the sex of the parents: Both mothers and fathers should be more distressed by infidelities that pose a greater threat to their genetic interests. Results were largely consistent with the predictions derived from evolutionary psychology: Both mothers and fathers regarded sexual infidelity as more distressing when committed by a daughter–in–law than by a son–in–law, and emotional infidelity as more distressing when it involved a son–in–law rather than a daughter–in–law. Discussion centered on the theoretical implications of research that examines the behavior of those who are, genetically, one step removed from the sexual behavior of others, but still affected by the reproductive consequences of that behavior.