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.”