The Trolley Problem
Overview
The Trolley Problem is a philosophical thought experiment that explores moral dilemmas involving life-and-death decisions. It challenges how we reason about ethics, particularly utilitarianism (maximising good outcomes) versus deontological ethics (following moral rules).
Origin
First introduced by Philippa Foot in 1967 and later expanded by Judith Jarvis Thomson, the Trolley Problem has become a central example in discussions of moral philosophy, ethics, and cognitive science.
The Basic Example
A trolley (heavy, unmanned metal cart) is out of control and heading down a track toward five people who are tied up and cannot move. You are standing next to a lever. If you pull it, the trolley will switch to another track where one person is tied up.
Do you pull the lever to save five people by sacrificing one?
Variations and Implications
Safe track
Instead of a single person being tied up on the track there is no one on the track, and the trolley will come to a safe stop if the lever is pulled.
- Implication: I use this version of the thought experiment when talking to people unused to addressing hypotheticals. In this example there seems to be no reason not to pull the lever. However, some people will still not pull the lever as they fear getting involved. They are worried they might not understand what's happening enough to take action.
Fat Man (Bridge) Variation
Instead of a lever, you're on a footbridge. You can stop the trolley by pushing a fat man off the bridge, killing him but saving five others.
Should you push the fat man off the bridge?
- Implication: Raises the issue of personal involvement and whether direct harm is morally different from indirect harm.
Loop Track
The trolley will loop back and kill the five unless diverted onto a loop track where a large person is tied. His body would stop the trolley.
Should you pull the lever and use the person's body to stop the trolley?
- Implication: In the original version, the solo individual is killed as a side-effect. The Loop Track makes the killing of the solo individual intentional; their body is the mechanism by which the trolley is stopped and the five are saved.
Transplant Case
A surgeon can save five patients by harvesting organs from one healthy person without consent. Some versions even make it clear that the healthy person has no family or friends who would miss him.
Should you take the organs from the healthy person?
- Implication: Tests the boundaries of utilitarian reasoning; killing one to save five in a medical context is widely rejected.
Self-Sacrifice
You realise with certainty that you can jump in front of the trolley yourself to stop it, dying to save five.
Should you jump?
Would you say someone else should have jumped if they, and not you, were in that situation?
- Implication: Raises the question of moral heroism and autonomy in self-sacrifice.
AI / Autonomous Cars
A self-driving car must choose between hitting five pedestrians or swerving to kill its driver.
Should it kill the pedestrians or the driver?
- Implication: Tests the real-world application of trolley logic in programming machines with ethical decision-making.
Hidden Information
You don’t know how many people are on the alternative track.
Should you pull the lever?
- Implication: Examines moral action under imperfect knowledge and risk assessment.
Probabilistic Trolley Problem: 10-or-None Version
A trolley is heading toward five people tied to the main track. If you pull the lever, there is a 50% chance that
- 10 people are tied on the side track — they will be killed.
- Or 0 people are on the side track — no one will be harmed.
You do not know which is the case.
Should you pull the lever?
- Implication: What are we prepared to risk for a perfect outcome?
The one's sacrifice
The one person has volunteered to be sacrificed to save five.
Should you pull the lever?
- Implication: Explores how consent alters the moral acceptability of sacrificing an individual.
The five's sacrifice
The five people have volunteered to be sacrificed to save one.
Should you pull the lever?
- Implication: Explores how consent alters the moral acceptability of sacrificing a group.
The Sympathetic Bystander
You may pull the lever, but the one person on the side track is your child or loved one, someone you know.
Should you pull the lever?
- Implication: Explores partiality, emotional bias, and whether personal attachments are morally relevant.
Old vs Young
Tied up on the track is a small child. If you pull the lever the trolley will kill five elderly men and women.
Should you pull the lever?
How would the numbers need to change for you to change your mind?
Healthy vs Unhealthy
On the track are five people who are heavy smokers and drinkers, and who generally lead an unhealthy lifestyle. On the other side is a single person who takes care of themselves and never drinks or smokes.
Should you pull the lever?
How would the numbers need to change for you to change your mind?
The Man in the Hammock
You may pull the lever, and the trolley will be diverted from the five tied up. No one is tied to the other track; however, after a short way, you know for certain that there is a gap in the tracks. The trolley will become derailed, roll down a small hill, and crash into a shed. The shed contains a man quietly sleeping in a hammock. The sleeping man will be killed.
Should you pull the lever?
- Implication: In the classic version, is there a sense that the people tied up on the track are somehow deserving of their fate? The man sleeping in the hammock is not "involved" in the scene; does that change anything?
Why I use it
Thought experiments like this one do not reveal moral facts, because there are none to reveal (see Objective morality). What they do is surface the hidden premises and competing interests inside our intuitions (Moral claims are incomplete assertions), and show how sensitive those intuitions are to framing, proximity, and consent. That sensitivity is itself evidence for the account of morality as a set of evolved and learned heuristics rather than a perception of moral law.