Give a stranger money with no way to make them pay you back, and most people send some
In the 1995 trust game, an 'investor' can send any share of a cash stake to an anonymous 'trustee,' and the amount sent triples before the trustee decides how much, if any, to send back. Pure self-interest predicts the investor sends nothing — but across repeated experiments, most investors send a substantial share, and most trustees send some of the tripled amount back.