Given total control over free money, most people still give some of it away

In the dictator game, one player decides alone how to split a sum of money with a second player, who has no say and no power to reject the split — unlike the related ultimatum game. Pure self-interest predicts the dictator keeps everything, yet across many studies a majority voluntarily give some money away, typically keeping around 70-80% for themselves. The result is widely cited evidence that fairness norms shape behavior even when there's no strategic reason to share at all.

— Wikipedia contributors, Dictator Game — Experimental economics