Last December, more than 1,000 customers at a Starbucks drive-thru window in Newington, Conn., did something irrational. They paid for the order of the person behind them in line. The money-saving move would be to break the chain: Gratefully acknowledge that the person ahead of you in line paid for your order, then drive off with your wallet firmly in your pocket, cackling madly like the Joker.

Why are so many people obeying the unwritten rules of the pay-it-forward movement? (By the way, Pay It Forward Day this year is April 24—no joke.) The usual explanation is that fairness is a product of altruism, the tendency to help other people at a cost to ourselves.