The trolley problem explained, with both answers

The trolley problem asks whether it is right to divert a runaway trolley onto a side track, killing one person to save five. A utilitarian says yes, because the outcome is better. Deontologists hold that some choices are forbidden whatever their results, and many of them treat this case differently from a close cousin in which you push a man off a bridge. Most people say yes to the lever and no to the push, and that difference is the real puzzle.

The case

A runaway trolley is heading for five people tied to the track. You stand by a lever. If you pull it, the trolley goes onto a side track where one person is tied. Do you pull it?

Philippa Foot introduced the case in 1967, in a paper on abortion and the doctrine of double effect. Judith Jarvis Thomson took it up in a 1976 article, gave it the name the trolley problem, and added a second version. In that one you stand on a bridge beside a large man. The only way to stop the trolley is to push him onto the track. Either way one person dies and four more live. The question is whether the two choices are the same.

What most people say

Wikipedia's article on the problem reports that about 90 percent of respondents in surveys chose to pull the switch. It also reports that a majority approve of pulling the switch to save a net of four lives, but disapprove of pushing the man to save the same net of four. In a survey of professional philosophers, it says, 68 percent would switch, 8 percent would not, and the remaining 24 percent had another view.

So most people do not treat the arithmetic as the whole story, and yet it is hard to say what else matters. That is why the problem has lasted.

The utilitarian answer

Utilitarianism is a kind of consequentialism, the view that, in the words of the Stanford Encyclopedia of Philosophy, whether an act is morally right depends only on the consequences of that act. Classic utilitarians put it as the greatest happiness for the greatest number. The Encyclopedia notes that this wording is misleading, since what counts is the total good minus the total bad.

On that test the lever and the bridge are the same. In both, one death replaces five, and the result is better. A consistent utilitarian therefore says to pull the lever and, if the case is just as described, to push the man too. The theory does not care about the route, only about the outcome.

The deontological answer

Deontology is, in the Encyclopedia's phrase, the kind of theory about which choices are morally required, forbidden or permitted. Its central claim is that some choices cannot be justified by their effects: no matter how good the consequences, some choices are forbidden.

Two ideas in the same entry help explain why many deontologists treat the lever and the bridge differently. The first is the doctrine of double effect. Deontologists are, the entry says, committed to something like it, which holds that we are categorically forbidden to intend evils such as killing the innocent. On one reading, the person on the side track is a foreseen result of pulling the lever, not the aim of it. On the bridge, the man's body is the means by which the trolley is stopped, so his death is part of the plan.

The second is a concern with using people. The entry describes a right against being used only as a means for producing good consequences without one's consent. The man on the bridge is used. The person on the side track, on this reading, is not.

Deontologists do not all agree about the lever. Some think that moving the trolley onto another track makes you a participant in the wrong, and say you should not pull it. The point is that the theory gives them room to treat the two cases differently, and utilitarianism does not.

Why the cases feel different

The Encyclopedia also separates killing from failing to prevent a death. Our categorical obligations, it says, are usually negative in content: we are not to kill. By that measure, doing something that kills a person weighs more than letting events take their course.

Whether such distinctions give a good reason, or only describe how we react, is still argued. A harder cousin of the case shows what is at stake. In it, a doctor could kill one healthy patient to save five patients who are dying. The Stanford Encyclopedia of Philosophy puts the problem for consequentialism this way: if one killing is worse than five deaths that do not involve killing, then the world in which the doctor does the transplant is worse than the world in which the doctor does not. One response from some consequentialists is to count a person's own killings as weighing more from that person's own point of view, which makes the transplant wrong even though it produces more good overall.

Where this leaves you

The trolley problem does not tell you what to do. It tells you what you are relying on when you decide. If you lean on results, you will pull the lever and probably push the man as well. If you lean on rules about what may be done to a person, you will draw a line somewhere between the two.

We set the two camps side by side on the page about utilitarianism versus Kantian deontology, and look at the underlying choice, results or rules, on the page about results or rules. You can read what each holds on its own page: utilitarianism and Kantian deontology. Philippa Foot has a page too, at Foot.

If you would like to see where your own answers to questions like this place you on the map, you can take the walk.