Individual differences are explicitly connected to social interaction in Darwin's notion of sexual selection. Traits that increase the probability of successful reproduction will tend to increase in frequency. This process operates partly through differential choice, by one sex, of certain traits in the other. According to the parental investment model, females frequently have more stringent criteria for the traits they will accept in a mate because they have a relatively larger investment in each offspring. Because human mating arrangements often involve a substantial commitment of resources by the male, it is necessary to invoke a distinction between the selectivity involved during casual mating opportunities and the selectivity exercised when choosing a long-term partner. Ninety-three undergraduate men and women rated their minimum criteria on 24 partner characteristics at four levels of commitment. In line with an unqualified parental investment model, females were more selective overall, particularly on status-linked variables. In line with a qualified parental investment model, males' trait preferences depended upon the anticipated investment in the relationship. Males had lower requirements for a sexual partner than did females, but were nearly as selective as females when considering requirements for a long-term partner.