Self-serving impulses and moral considerations often act as two opposing forces that govern our everyday behavior. But how does the brain decide which one wins?

As a step toward answering that question, Ernst Fehr of the University of Zurich and his colleagues watched as 52 volunteers played the ultimatum game, an anonymous exchange in which an individual decides whether to punish a partner's behavior at his or her own cost. In this version of the game, one player proposes how to divide 20 Swiss francs with the second player. If the responder accepts, the first player gets the money demanded and the responder gets the rest. But if the responder rejects a lopsided offer, neither gets anything. “In this game, players must overcome selfish impulses if they want to punish their partners for an unfair offer,” Fehr explains.

To test how the brain regulates the control of these impulses, the researchers disrupted the activity of the right or left dorsolateral prefrontal cortex (DLPFC), a brain region known to be involved in self-control. Using repetitive transcranial magnetic stimulation, a technique that delivers short magnetic pulses that penetrate the skull and temporarily disrupt neural firing, the team inhibited the DLPFC in 36 responders while they were making their decision. The remaining 16 players served as a control group.

The researchers found that turning off the right side of the DLPFC made the volunteers much more likely to accept highly uneven splits, even though they still judged these offers as unfair. “That suggests that right DLPFC activity is crucial when it comes to the ability to override selfish impulses,” says Fehr, adding that dysfunctions of this brain region may cause certain mental disorders characterized by excessive selfishness.