The Trolley Problem: Moral Dilemmas, Double Effect, and the Limits of Consequentialism

A runaway trolley is hurtling toward five people tied to the track. You are standing beside a lever that can divert it onto a side track — where there is only one person. Pull the lever? Most people say yes. Now a different scene: there is no lever, but you are on a footbridge beside a large man; pushing him onto the track would stop the trolley and save the five. Most people, now, refuse. Why is five-for-one acceptable in one case and repugnant in the other, if the arithmetic is the same? This is the most famous form of the trolley problem — perhaps the best-known thought experiment in ethics.

1. The original case: Philippa Foot

The dilemma is born in a 1967 essay by Philippa Foot, “The Problem of Abortion and the Doctrine of Double Effect.” Her real target was not trolleys but the distinction between causing a harm intentionally and merely foreseeing it as a side effect. Foot imagined the driver of a brakeless trolley who can steer from five onto one — and contrasted the case with that of a judge tempted to condemn an innocent man to calm a mob. The intuition that steering is acceptable but condemning the innocent is not suggests that it is not only the outcome that counts.

2. Thomson and the variations

It was Judith Jarvis Thomson — alongside other analytic philosophers — who named the “trolley problem” (in “Killing, Letting Die, and the Trolley Problem,” 1976) and multiplied its variations in the canonical essay “The Trolley Problem” (1985). The decisive contrast is between the Bystander at the Switch (pull the lever: most approve) and the Footbridge (push the man: most condemn). Add the case of the transplant surgeon, who could kill one healthy patient to distribute his organs and save five: almost no one accepts it, though the balance is identical. The variations (the “loop,” the “ricochet”) were designed to isolate exactly which factor makes the intuition flip.

3. The distinctions at stake

The trolley problem is an instrument for testing three distinctions that pure consequentialism tends to erase:

• Killing vs. letting die (act vs. omission): is there a moral difference between bringing about a death and allowing one to occur?
• Intention vs. foresight: diverting the trolley foresees the death of the one but does not intend it (if he stepped off the track, all the better); pushing the man on the footbridge uses his death as a means.
• Using as a means vs. collateral harm: here Kant’s formula resonates — to treat a person merely as a means is what makes the Footbridge so repugnant.

4. The doctrine of double effect

These intuitions were systematized, centuries earlier, by the doctrine of double effect, whose root lies in Thomas Aquinas (in his discussion of self-defense) and which Elizabeth Anscombe revived in the twentieth century. On it, it is permissible to perform an act with a foreseen bad effect provided that: (1) the act itself is good or neutral; (2) the bad effect is not intended, only tolerated; (3) the good effect is not produced by means of the bad one; and (4) there is proportion between the two. By the DDE, diverting the trolley passes the test (the one’s death is collateral); pushing the man does not (his death is the means). The doctrine is central to bioethics (see bioethics) and to the ethics of war, where it distinguishes collateral civilian casualties from intentional attacks on civilians.

5. The empirical turn: moral psychology

From the 2000s on, the psychologist Joshua Greene took the dilemmas into the laboratory. Using fMRI, he observed that “personal” dilemmas like the Footbridge (in which one inflicts harm with one’s own hands) activate regions tied to emotion more strongly, whereas “impersonal” dilemmas like the switch activate regions tied to cold reasoning. Hence his dual-process theory: our deontological judgments would be, in large part, automatic emotional alarms, and our utilitarian ones the fruit of deliberate calculation.

Caution is in order: even if this explains why we feel what we feel, it does not settle what is right — to conflate the planes would be to commit a fallacy (from the is to the ought). The psychological description of an intuition neither justifies nor refutes it.

6. Thomson’s about-face

Thomson herself, in “Turning the Trolley” (2008), changed her mind: she came to hold that, after all, it is not permissible to divert the trolley. Her argument: if you would not be willing to sacrifice yourself (diverting the trolley onto yourself, were that an option), you have no right to redirect the threat onto a third party who never asked to be in the game. The recantation shows how the “obvious” verdict in the switch case is itself open to dispute.

7. Contemporary applications

Far from a mere puzzle, the problem reappears in real decisions. Self-driving cars need rules for cases of unavoidable accident — MIT’s Moral Machine project gathered millions of judgments about whom a vehicle should “spare.” Medical triage in disasters, the allocation of scarce resources, and the law of war all face versions of the same dilemma. There are also critics of “trolleyology”: for them, such artificial cases would distort moral deliberation, which is always situated and concrete — an objection that stands as a methodological warning.

8. Assessment

The trolley problem is not, at bottom, about trolleys: it is an instrument for asking whether morality reduces to maximizing good outcomes (the wager of utilitarianism) or whether there are limits that the arithmetic of consequences cannot cross — rights, intentions, the difference between doing and allowing (the wager of Kant’s deontology). That our intuitions resist a single formula is, perhaps, the most philosophical datum of all: it reveals that the moral life is richer than any theory taken in isolation — and that to think it through requires, precisely, cases that force us to make explicit what, in general, we feel without knowing why.

Essential reading

• Philippa Foot, “The Problem of Abortion and the Doctrine of Double Effect” (1967).
• Judith Jarvis Thomson, “The Trolley Problem” (1985) and “Turning the Trolley” (2008).
• Elizabeth Anscombe, “Modern Moral Philosophy” (1958).
• Joshua Greene, Moral Tribes (2013).

← Articles