For more than 20 years, researchers have been asking the question: Why are people so nice to strangers but so mean to people that they know? Evolutionarily, it doesn’t make much sense. However, data has consistently shown people’s willingness to warm up to strangers and punish those they know have a bad reputation.

Over the years, evolutionary psychologists have created two opposing theories to explain the phenomenon. One theory, called Social Exchange Theory, reasons that our ancestors lived in small communities and therefore were likely to encounter anyone they met more than once. Gaining the trust of strangers is evolutionarily advantageous because it creates the opportunity to foster a mutually beneficial relationship. Conversely, if it is known that someone has a history of cheating, it is unlikely that trusting them will be beneficial.

The opposing theory is the Group Cooperation Theory. It says that trust and kindness within a group is evolutionarily advantageous because a group plagued with derisiveness and mistrust is unlikely to survive and reproduce. Under this theory, individuals with a bad reputation would be punished for causing rancor within the group.

Past data has been used to support both theories. Max Krasnow, Leda Cosmides, and John Tooby, researchers at UCSB’s Center for Evolutionary Psychology, set out to pit the theories against each other in a study published recently.

Participants in the study interacted with others whom they believed to be real people but were actually computers following a script. Test subjects invested fake money with the computer simulators, and their investments would either be returned with interest or stolen from them. Predictably, the participants were likely to reinvest their money with those who returned their investment and unlikely to reinvest with those who stole their money. Also, participants were unlikely to invest with others whom they knew had stolen from others. However, even when the computer character had a history of stealing from others, participants would still invest with them if they had reliable information that they wouldn’t be cheated.

The researchers concluded that their data supports the Social Exchange Theory. They reason that if the participants were trying to preserve the unity of the group, they would punish cheaters despite the possibility of individual benefits.

Overall, the study showed that humans have highly evolved mechanisms for interacting with others and knowing whom to trust. These mechanisms surely contribute to a harmonious society. Unfortunately, empirical data suggests that our kindness is ultimately motivated by selfishness.


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