Reciprocation was a big deal while we were evolving — if we scratch someone’s back, we expect it to be returned at some point. This is one of the reasons that negative gossip evolved, to illuminate the rascals who don’t return favours to your fellow tribe members.

You might be interested in some of Mark Wright’s books, he wrote a lot about this kind of thing (evolutionary game theory).

I write about psychology, philosophy, and society. Also a part-time moose masseuse. Hit me up if you need me to de-knot your elk.