Theoretical analyses and studies with children suggest that females are more likely than males to respond to threats of social exclusion with exclusion. Here we present a series of studies using a modified version of a computerized competitive game that participants play against two fictitious opponents. In previous studies, females and males have typically made identical strategy choices when playing this game. We show that when players are told that the two fictitious opponents may form an exclusionary alliance against them, females modify their competitive strategies by forming more preventive exclusionary alliances than males do. These results support the idea that adult females are more likely than males to form preventive exclusionary alliances when faced with a social threat. The results further suggest that females and males compete in different ways.