What's best for the individual and what's best for society are often not the same thing--this predicament is the premise for the famous "prisoner's dilemma" game. However, healthy societies depend on individuals cooperating for the common good, even at the risk of personal loss. In theory, individuals should choose what's in their own best self-interest, but the reality is that many people--and even animals--instead choose to cooperate, to the puzzlement of many professionals who study sociology, game theory and other disciplines.