Offer someone free money on unfair terms, and they'll refuse it out of spite
In the ultimatum game, one player proposes how to split a sum and the other can accept or reject, but rejecting means both get nothing. Rational self-interest predicts any positive offer gets taken; real players routinely reject offers below roughly 20-30%, giving up cash just to punish unfairness. First run in 1982, it's one of the most replicated results in behavioural economics.