Reciprocity is so fundamental to nature that people can’t stop doing it:
Most of the studies about “people are irrational” are times psychologists fail to realize that people are acting on an iterated (rather than single round) game — and enforcing reciprocity through their “irrational” acts.
We know at an instinctive level that reciprocity is necessary in game theory — eg, Iterated Prisoner’s Dilemma.