One of the most powerful rules of interpersonal behavior is that people are kinder to members of their in-groups than to members of their out-groups. Are people also kinder to their future selves when they expect them to remain members of their current in-groups rather than become members of their current out-groups? In 2 studies, participants in an emotionally charged debate expected either to remain on the same team or to join the opposing team when they returned the following week. Those who expected to join the opposing team were less willing to sacrifice for their future selves, leaving more of an unpleasant task for their future selves to finish and treating their future selves as unkindly as they treated a stranger. These results suggest that the rules that govern interpersonal behavior may also govern intertemporal behavior, and suggest new strategies to encourage prudent decisions.