If you got a Christmas card in the mail from a complete stranger, would you send them one back?
Surprisingly, this isn’t a purely hypothetical question. Social psychologists have used Christmas cards from a stranger as a model to research the ‘reciprocity norm’ – the expectation that you should return a favor, and help someone who helps you.
In 1976, researchers Phillip Kunz and Michael Woolcott sent cards to a random sample of 578 Americans. Overall, they found that 20% of the recipients