Reciprocal Altruism

the moral emotion of "reciprocal altruism," which evolved over the Paleolithic eons to demand fairness on the part of our potential exchange partners. "I'll scratch your back if you'll scratch mine" only works if I know you will respond with something approaching parity. The moral sense of fairness is hard-wired into our brains and is an emotion shared by most people and primates tested for it, including people from non-Western cultures and those living close to how our Paleolithic ancestors lived.

Why people believe weird things about money - Los Angeles Times: