Consider an experiment economists call “the ultimatum game”: The experimenter gives one player, the sender, $20 to distribute between himself and another player, the receiver. An egalitarian sender might propose a split of $10 each. A more selfish sender might propose to give the receiver only $1, keeping $19 for himself. If the receiver accepts the deal, the two players collect their shares. If the receiver rejects the deal, both walk away with nothing. Were humans perfectly rational, the receiver would accept whatever is offered: even a dollar is better than nothing, right? Instead, researchers find, receivers will reject an overly lopsided deal, gladly giving up their shares just to punish the stingy senders.