HOW people collaborate, in the face of numerous temptations to cheat, is an important field of psychological and economic research. A lot of this research focuses on the “tit-for-tat” theory of co-operation: that humans are disposed, when dealing with another person, to behave in a generous manner until that other person shows himself not to be generous. At this point co-operation is withdrawn. Fool me once, in other words, shame on you. Fool me twice, shame on me. Originally published on November 19, 2011.