henry copeland:

People who cunningly use cooperation and egoism are 'unbeatable' -- ScienceDaily

However, if one of the players was additionally enticed with a bonus, this player would turn into an extortioner in many cases. Despite the fact that the other player would keep trying to discipline them by refusing cooperation, the extortioner would resist and cooperate even less rather than more frequently over the course of the experiment. Extortioners were also shown to be most successful in the long-term, even in the experiment in which the potential bonus player was not predetermined.
- www.sciencedaily.com