The story has a sequel. In 1989, the Polish mathematician Martin Nowak produced a programme that beats Tit-for-Tat. He called it Generous. It overcame one weakness of Tit-for-Tat, namely that when you meet a particularly nasty opponent, you get drawn into a potentially endless and destructive cycle of retaliation, which is bad for both sides. Generous avoided this possibility by randomly but periodically forgetting the last move of its opponent, thus allowing the relationship to begin again. What Nowak had produced, in fact, was a computer simulation of forgiveness