When we condemn the behavior of a politician, celebrity, or friend, we often end up appealing to our moral intuitions. “It just feels wrong!” we say. But where do these intuitive judgments come from? Are they reliable moral guides? Recently, some unusual research has raised new questions about the role of intuitive responses in ethical reasoning. Joshua Greene, a philosophy graduate now working in psychology who has recently moved from Princeton University to Harvard, studied how people respond to a set of imaginary dilemmas. In one dilemma, you are standing by a railroad track when you notice that a trolley, with no one aboard, is heading for a group of five people. They will all be killed if the trolley continues on its current track. The only thing you can do to prevent these five deaths is to throw a switch that will divert the trolley onto a side track, where it will kill only one person. When asked what you should do in these circumstances, most people say that you should divert the trolley onto the side track, thus saving a net four lives. In another dilemma, the trolley, as before, is about to kill five people. This time, however, you are not standing near the track, but on a footbridge above the track. You cannot divert the trolley. You consider jumping off the bridge, in front of the trolley, thus sacrificing yourself to save the five people in danger, but you realize that you are far too light to stop the trolley. Standing next to you, however, is a very large stranger. The only way you can prevent the trolley from killing five people is by pushing this large stranger off the footbridge, in front of the trolley. If you push the stranger off, he will be killed, but you will save the other five. When asked what you should do in these circumstances, most people say that it would be wrong to push the stranger. This judgment is not limited to particular cultures. Marc Hauser, at Harvard University, has put similar dilemmas on the web in what he calls a “Moral Sense Test,” available in English, Spanish, and Chinese ( http://moral.wjh.harvard.edu ). After receiving tens of thousands of responses, he finds remarkable consistency despite differences in nationality, ethnicity, religion, age, and sex. Philosophers have puzzled about how to justify our intuitions in these situations, given that in both cases, the choice seems to be between saving five lives at the cost of taking one life. Greene, however, was more concerned to understand why we have the intuitions, so he used functional Magnetic Resonance Imaging, or fMRI, to examine what happens in people’s brains when they make these moral judgments. Greene found that people asked to make a moral judgment about “personal” violations, like pushing the stranger off the footbridge, showed increased activity in areas of the brain associated with the emotions. This was not the case with people asked to make judgments about relatively “impersonal” violations like throwing a switch. Moreover, the minority of subjects who did consider that it would be right to push the stranger off the footbridge took longer to reach this judgment than those who said that doing so would be wrong. Why would our judgments, and our emotions, vary in this way? For most of our evolutionary history, human beings – and our primate ancestors – have lived in small groups, in which violence could be inflicted only in an up-close and personal way, by hitting, pushing, strangling, or using a stick or stone as a club. To deal with such situations, we developed immediate, emotionally based intuitive responses to the infliction of personal violence on others. The thought of pushing the stranger off the footbridge elicits these responses. On the other hand, it is only in the last couple of centuries – not long enough to have any evolutionary significance – that we have been able to harm anyone by throwing a switch that diverts a train. Hence the thought of doing it does not elicit the same emotional response as pushing someone off a bridge. Greene’s work helps us understand where our moral intuitions come from. But the fact that our moral intuitions are universal and part of our human nature does not mean that they are right. On the contrary, these findings should make us more skeptical about relying on our intuitions. There is, after all, no ethical significance in the fact that one method of harming others has existed for most of our evolutionary history, and the other is relatively new. Blowing up people with bombs is no better than clubbing them to death. And surely the death of one person is a lesser tragedy than the death of five, no matter how that death is brought about. So we should think for ourselves, not just listen to our intuitions.