The Fat Man Variant of the Trolley Problem

Direct Answer

The scenario: a runaway trolley is hurtling toward five people who will be killed unless it is stopped. You are standing on a footbridge above the track, and beside you stands a large man. The only way to stop the trolley is to push the man off the bridge so that his body, landing on the track, blocks the trolley. He will be killed; the five will be saved. Is it permissible to push him?

Almost everyone says no. This is the fat man variant of the trolley problem, introduced by Judith Jarvis Thomson in her 1976 paper "Killing, Letting Die, and the Trolley Problem." Its power lies in the contrast with the standard switch case: when a lever can divert the trolley onto a side track where one person will die, most people approve pulling the lever. But when saving the five requires using a person's body as an instrument, the verdict flips. The fat man variant therefore isolates what philosophers call the distinction between doing harm and allowing harm, and between using someone as a means and harming them as a foreseen side effect.

Historical Context

Philippa Foot invented the trolley problem in her 1967 paper "The Problem of Abortion and the Doctrine of the Double Effect" to test whether the distinction between killing and letting die could do real moral work. Her version contrasted a driver who turns the trolley with a bystander who diverts it. Thomson saw that Foot's distinction could not bear the weight, and she sharpened the problem by adding the fat man: a case in which saving the five requires positively shoving a person to his death. Thomson's fat man quickly became the canonical version of the dilemma, the one that appears in ethics textbooks, psychology experiments, and public debates about everything from self-driving cars to military robots.

Philosophical Analysis & Variants

The fat man case is a precision instrument for testing moral theories. The utilitarian, who judges actions by their outcomes, sees no difference between the switch case and the bridge case: in both, one dies so that five live, so both are permissible and even obligatory. The Kantian deontologist judges the push wrong because it uses the man merely as a means: his body is treated as a tool for stopping the trolley, and he is not given the respect owed to persons.

The classic middle-ground explanation is the doctrine of double effect: an act that brings about a foreseen bad side effect can be permissible if the bad effect is not intended, while an act that intends harm as a means is wrong. In the switch case, the bystander intends to save the five and merely foresees the death of the one on the side track. In the fat man case, the death of the man is not a side effect; it is the very means by which the trolley is stopped. The fat man variant thus exposes the intuition that intention matters, not just outcome.

Thomson's own conclusion is more radical. She argued that the doctrine of double effect cannot explain the cases, and that the real work is done by people's rights: the fat man has a right not to be pushed, a right that is not overridden merely because pushing him would save five others. The fat man variant therefore supports a rights-based, deontological reading of the dilemma. Related variants sharpen the picture further: in the loop variant, the trolley is diverted onto a track that loops back; the one person will be killed only if his body stops the trolley before it rejoins the main track and kills the five. The loop variant shows that even a "redirect" can be a case of using someone as a means, undermining simple consequentialist answers.

Contemporary Relevance

The fat man variant has moved out of the philosophy seminar and into engineering and policy. Designers of autonomous vehicles must program crash algorithms that decide whom to sacrifice, and surveys repeatedly show that people approve "switch"-style programming but recoil from "push"-style design even when the outcomes are identical. Military ethicists debate whether autonomous drones may target a vehicle knowing a civilian will die, or only when the civilian's death is the intended means of attack. In moral psychology, the contrast between the switch and the fat man is one of the most robust findings in the field, and neuroscientists have used it to study how emotion and reason compete in moral judgment. The fat man variant remains the clearest single illustration of the gap between what maximizes welfare and what respects persons.

Related Concepts

The fat man variant belongs to the wider family of moral dilemmas. Compare it with the transplant dilemma, in which a surgeon must decide whether to kill one healthy patient to save five dying ones, and with the loop variant. The underlying machinery is the doctrine of double effect, and the competing frameworks are deontology and utilitarianism. The thinkers who defined the problem are Philippa Foot and Judith Jarvis Thomson.

Further Learning

• Stanford Encyclopedia of Philosophy, "The Trolley Problem": https://plato.stanford.edu/entries/trolley-problem/
• Stanford Encyclopedia of Philosophy, "The Doctrine of Double Effect": https://plato.stanford.edu/entries/double-effect/
• Stanford Encyclopedia of Philosophy, "Moral Dilemmas": https://plato.stanford.edu/entries/moral-dilemmas/