By the time Greene came to Princeton for graduate school in 1997, however, he had become dissatisfied with utilitarians and Kantians alike. None of them could explain how moral judgments work in the real world. Consider, for example, this thought experiment concocted by the philosophers Judith Jarvis Thompson and Philippa Foot: Imagine you’re at the wheel of a trolley and the brakes have failed. You’re approaching a fork in the track at top speed. On the left side, five rail workers are fixing the track. On the right side, there is a single worker. If you do nothing, the trolley will bear left and kill the five workers. The only way to save five lives is to take the responsibility for changing the trolley’s path by hitting a switch. Then you will kill one worker. What would you do?

 

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You are running down a crowded corridor in the airport, trying to catch a flight that’s about to leave. Suddenly, an old woman in front of you slips and falls hard.


Do you stop to help, knowing that you’ll miss your plane?

Now imagine that you are watching the runaway trolley from a footbridge. This time there is no fork in the track. Instead, five workers are on it, facing certain death. But you happen to be standing next to a big man. If you sneak up on him and push him off the footbridge, he will fall to his death. Because he is so big, he will stop the trolley. Do you willfully kill one man, or do you allow five people to die?

Logically, the questions have similar answers. Yet if you poll your friends, you’ll probably find that many more are willing to throw a switch than push someone off a bridge. It is hard to explain why what seems right in one case can seem wrong in another. Sometimes we act more like Kant and sometimes more like Mill. “The trolley problem seemed to boil that conflict down to its essence,” Greene says. “If I could figure out how to make sense of that particular problem, I could make sense of the whole Kant-versus-Mill problem in ethics.”

The crux of the matter, Greene decided, lay not in the logic of moral judgments but in the role our emotions play in forming them. He began to explore the psychological studies of the 18th-century Scottish philosopher David Hume. Hume argued that people call an act good not because they rationally determine it to be so but because it makes them feel good. They call an act bad because it fills them with disgust. Moral knowledge, Hume wrote, comes partly from an “immediate feeling and finer internal sense.”

Moral instincts have deep roots, primatologists have found. Last September, for instance, Sarah Brosnan and Frans de Waal of Emory University reported that monkeys have a sense of fairness. Brosnan and De Waal trained capuchin monkeys to take a pebble from them; if the monkeys gave the pebble back, they got a cucumber. Then they ran the same experiment with two monkeys sitting in adjacent cages so that each could see the other. One monkey still got a cucumber, but the other one got a grape—a tastier reward. More than half the monkeys who got cucumbers balked at the exchange. Sometimes they threw the cucumber at the researchers; sometimes they refused to give the pebble back. Apparently, De Waal says, they realized that they weren’t being treated fairly.

In an earlier study, De Waal observed a colony of chimpanzees that got fed by their zookeeper only after they had all gathered in an enclosure. One day, a few young chimps dallied outside for hours, leaving the rest to go hungry. The next day, the other chimps attacked the stragglers, apparently to punish them for their selfishness. The primates seemed capable of moral judgment without benefit of human reasoning. “Chimps may be smart,” Greene says. “But they don’t read Kant.”

The evolutionary origins of morality are easy to imagine in a social species. A sense of fairness would have helped early primates cooperate. A sense of disgust and anger at cheaters would have helped them avoid falling into squabbling. As our ancestors became more self-aware and acquired language, they would transform those feelings into moral codes that they then taught their children.

This idea made a lot of sense to Greene. For one thing, it showed how moral judgments can feel so real. “We make moral judgments so automatically that we don’t really understand how they’re formed,” he says. It also offered a potential solution to the trolley problem: Although the two scenarios have similar outcomes, they trigger different circuits in the brain. Killing someone with your bare hands would most likely have been recognized as immoral millions of years ago. It summons ancient and overwhelmingly negative emotions—despite any good that may come of the killing. It simply feels wrong.

Throwing a switch for a trolley, on the other hand, is not the sort of thing our ancestors confronted. Cause and effect, in this case, are separated by a chain of machines and electrons, so they do not trigger a snap moral judgment. Instead, we rely more on abstract reasoning—weighing costs and benefits, for example—to choose between right and wrong. Or so Greene hypothesized. When he arrived at Princeton, he had no way to look inside people’s brains. Then in 1999, Greene learned that the university was building a brain-imaging center.