A stranger gets a pot of money and offers you a share of it. If you accept the offer, both of you walk away with your proposed shares. If you reject, you both leave with nothing.

This is the ultimatum game—a classic psychological experiment used to study fair play. If both players behave completely selfishly, the proposer might offer as little as possible, while the responder should accept any offer as long as it’s not zero. That way, both of them walk away with something. In practice, responders typically reject any offers less than 20 percent. They care enough about fairness to do themselves out of money in order to punish unfair partners. Meanwhile, proposers from industrialised countries, who are