The trolley problem began with a tram driver

The runaway trolley is the best-known dilemma in ethics. It comes from a 1967 paper by Philippa Foot about abortion, and her question was not quite the one people ask now.

A runaway trolley is heading for five people on the track. You can pull a lever and send it onto a side track, where it will kill one person instead. What should you do?

The scenario is usually traced to the British philosopher Philippa Foot. In 1967 she published a paper in the Oxford Review called The Problem of Abortion and the Doctrine of the Double Effect. The runaway vehicle was one of several cases she used to think about when it is acceptable to cause harm.

Five men are working on one track and one man on the other.

In Foot's version the person deciding is the driver of a runaway tram, not a bystander. She set it beside another case: a judge who could stop rioters from taking bloody revenge only by framing and executing an innocent man. In both, one life is traded for five. Yet most people feel the driver should steer towards the one, and that the judge must not frame anyone.

Foot's answer drew on a difference between duties. The duty not to harm people is stronger than the duty to help them. The driver will harm someone whichever way he steers, so the better choice is to harm fewer. The judge would be harming an innocent person to protect others, and that is a different matter.

From driver to bystander

The name came later. Judith Jarvis Thomson called it the trolley problem in a 1976 article, and in 1985 she gave the lever to a bystander. Others added variants. In the best known, you stand on a footbridge and can stop the trolley only by pushing a heavy man into its path. Most people who would pull the lever say they would not push him, though the arithmetic is the same. Explaining that gap is the real trolley problem. Thomson herself later argued, in 2008, that the bystander should not switch at all.

Foot wasn't the first to imagine such a choice. In 1905 Frank Chapman Sharp put a version in a moral questionnaire for students at the University of Wisconsin, in which the one person on the other track was the switchman's own child.

Today the problem turns up in brain imaging studies and in debates about self-driving cars. Its original purpose was narrower: to find the principle that makes some trades of one life for five feel acceptable and others plainly wrong.