the bank. Taking money away from the donor is a form of costly punishment. It is costly in two ways: the punisher loses money he could have kept, and the donor loses money that he unfairly kept in his previous exchange. When donors kept a significantly larger portion of the original sum, observers punished, paying the costs. They also reported feeling good about taking down the cheapskates. Where was this honey hit to the brain coming from? To figure this out, Fehr and his colleagues put people in a brain scanner and used a technique called Positron Emission Tomography or PET. This type of scanning provides a picture of how much glucose is used up by different brain areas during a task. Higher glucose consumption occurs when there is higher activity in a brain region. When punishers decided to punish a selfish donor, glucose consumption increased in a region of the brain associated with reward: the dorsal striatum. This region is also active when you eat ice cream, earn money, and solve an unexpected problem. Punishers incurred a financial cost, but gained emotional elation and internal reward. It turns out that giving someone his just deserts feels like eating dessert, but without the caloric gain. When distinctively promiscuous punishment evolved, it transformed our capacity to cooperate and to maintain conformity to social norms. It provided us with the tools to not only repair a puncture in the system of norms, but to feel good about the costs personally incurred. When we punish, we have served justice and served ourselves a helping of the brain’s rewards. The fossil record doesn’t capture when we evolved the capacity to punish, as skulls and bones, stone tools, and even paintings are silent on why someone received a spear through the head — perhaps punished for a wrong doing or perhaps an enemy, a competitor interested in the same resources, or a suspected lover. No one will ever know. What we do know is that other primates never punish cheaters who break th