A September 23 article in the Boston Globe by Drake Bennett called Bad sports: Why people cheat when they don't have to. He mentions some research by economists and psychologists. One thing that kept coming up is that people will cheat if they think others are cheating and that if they don't cheat, they will be at a competitive disadvantage. Like if players on other teams are taking steroids, if you don't, you will have a harder time winning. So you are just trying to level the playing field.
Other issues that come up in the article include the possibility that favored teams in NCAA basketball win by less than expected, implying that the winning players hold down the score and take payoffs from gamblers, vote trading among figure skating judges, that people will cheat less if they know others are not cheating, sometimes people cheat even if they know there is a chance they will get caught and that if people have to write down as many of the Ten Commandments as they could remember before taking a test, it virtually eliminated cheating.