Evolutionary biologists argue that our tendency to engage in reciprocal behavior is a natural product of evolution. You are more likely to survive if you receive help from others. And you are more likely to receive that help if you have offered assistance in the past. So the genes that encode the reciprocal instinct were more likely to be passed on. And thus the fact that the human species has made it to now is directly dependent on our building social interactions that are reliable, useful, and trustworthy.
