As you report, Nicholas Christakis and his colleagues at Harvard University are studying how people cooperate in the face of the temptation to cheat (“Make or Break?” November 19, 2011). It is well known that humans are disposed to “tit-for-that” behavior. They behave generously toward others until others show themselves not to be generous. At his point cooperation is withdrawn. When defectors learn the error of their ways, they strive to cooperate more actively. Christakis and his team used a public-goods game for their experiment with social networks on the World Wide Web. By and large, the research supports the tit-for-that theory of cooperation. In particular, it shows that social retaliation has a marked effect on defectors. It would be useful to extend this research to organized crime, where defectors are punished more harshly for uncooperative behavior than is usually the case. Mafias are famous for cooperation, and a good deal can be learned from them about how humans cooperate in general. Also, there are many social networks on the web that simulate organized crime. Such games appear to be quite popular, too. It may well be that early humans were not too generous toward defectors, either. Primates surely are not.