Why are some people always jerks? Yale University psychologists have developed a mathematical model that provides an answer, and also helps to explain why the rest of us are usually nice, even to strangers.

The model developed by Yale's Adam Bear and David Rand incorporates ideas from the evolutionary game theory of cooperation and the behavioral economics of intuition and deliberation. Participants play games where they can be either helpful or selfish, and make choices using rules of thumb or careful reasoning. Some games have no chance of a future payoff for being nice; other games allow for the possibility of reciprocation.

This is what the model predicts will happen: People who come from a supportive and friendly environment learn intuitively to cooperate—even with strangers where there is no potential payoff—because they have often benefited from such generous behavior. However, if they take time to deliberate, they overrule their cooperative instinct if they realize there is no possibility of future payoff.

People who are typically surrounded by jerks, on the other hand, learn intuitively to be selfish—and also learn not to deliberate. So, the model shows, they wind up acting selfishly even when cooperating would actually pay off, because they don't stop to think.