Trivers argues that friendship and various social emotions evolved in order to manage reciprocity. Liking and disliking, he says, evolved to help present day humans' ancestors form coalitions with others who reciprocated and to exclude those who did not reciprocate. Moral indignation may have evolved to prevent one's altruism from being exploited by cheaters, and gratitude may have motivated present day humans' ancestors to reciprocate appropriately after benefiting from others' altruism. Likewise, present day humans feel guilty when they fail to reciprocate. These social motivations match what evolutionary psychologists expect to see in adaptations that evolved to maximize the benefits and minimize the drawbacks of reciprocity.