Human societies are organized around cooperative interactions. But why would natural selection equip selfish individuals with altruistic tendencies? This question has fascinated evolutionary ...

Humans are the champions of reciprocity. Experiments and everyday experience alike show that what Adam Smith called ‘our instinct to trade, barter and truck’ relies to a considerable extent on the ...

In Influence: The Psychology of Persuasion, Robert Cialdini discusses the concept of reciprocity, which is that we are hard-wired to respond positively to a gift, even if we didn’t ask for it, or even ...