“Of course I will help Caleb put up posters when he runs for student body president,” Stella says. “I’m going to need his help later this year when the marching band holds its annual car wash.” What kind of altruism is Stella practicing here?

Stella is helping Caleb now because she expects him to help her later with the marching band’s car wash. This is reciprocity: helping someone with the expectation that they will return the favor. Kin selection involves helping relatives, while internal rewards are personal feelings gained from helping.

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