Most the­o­ries of group for­ma­tion argue that people cluster together based on pre-​​existing similarities—shared eth­nicity or beliefs or a favorite team. “If you asked the average person on the street or even many psy­chol­o­gists to pick one thing that could explain how groups form,” says Lisa Bar­rett, a psy­chol­o­gist at North­eastern Uni­ver­sity in Boston who was also not involved in the research, “I’m not sure they would pick helping and harming. I think that’s pretty significant.”

It may be that groups are main­tained and elab­o­rated with more com­pli­cated processes related to iden­tity and cul­tural prac­tices,” Bar­rett adds. “But what this shows is that simple affec­tive reac­tions to helping and harming are suf­fi­cient to model group formation.”

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