The weight of existing evidence suggests that men display greater upset in response to a long-term partner's sexual infidelity, whereas women display greater upset in response to a partner's emotional infidelity. This sex difference was first hypothesized by evolutionary psychologists, who argued that the difference may reflect sex-differentiated evolved psychological design. Some socialization theorists, in contrast, have argued that the difference may be attributable to sex-differentiated socialization practices. A. Fenigstein and R. Peltz (2002) collected data from parents of undergraduates about upset in response to a child's partner's infidelities and found that both sexes report greater upset in response to a son's partner's sexual infidelity and in response to a daughter's partner's emotional infidelity. The key variable therefore is the sex of the child, as predicted from a heuristic application of an evolutionary perspective, and not the sex of the parent, as predicted from a heuristic application of one socialization perspective. We report a replication of these findings using data collected from retirees with an average age of about 70 years who have at least one son and one daughter and most of whom have grandchildren. Copyright © 2004 John Wiley & Sons, Ltd.