How effective are reward and punishment in enforcing cooperation?

Reward and punishment can help to enforce group cooperation. Yet, this requires that humans direct their rewards to cooperators and their punishments to non-cooperators, both indiscriminately and exclusively. Whenever they, for example, punish some non-cooperators more than others or altogether condition punishment on other factors than lack of cooperation, punishment may lose its effectiveness to enforce cooperation. My research is aimed at answering the questions of how people use reward and punishment, and what consequences this may have on their cooperation-enhancing potential.