Friday, 7 December 2012

Thought Experiment #1: The Runaway Trolley Car


Thought experiments are used in philosophy and ethics, philosophy, and even science as a way of clarifying and testing concepts and theories. For example, ethicists can use thought experiments to present a hypothetical dilemma, and examine the most “intuitive” response, drawing out implications for ethical issues in the real-world.

I thought (no pun intended) it might be fun to blog some well-known thought experiments, hopefully as part of an occasional series. As well as examining your own ethical reasoning processes, thought experiments are also useful as a revision exercise: you use them to test out and evaluate the ethical theories you are studying in your A level.

 One of the most famous thought experiments is The Runaway Trolley Car. It works like this:

A runaway trolley car is hurtling down a track. In its path are five people who will definitely be killed unless you, a bystander, flip a switch which will divert it on to another track, where it will kill one person. Should you flip the switch?
So have a think – what would you do?

Our second thought experiment develops this scenario a little. It’s called The Fat Man and the Trolley Car.

The runaway trolley car is hurtling down a track where it will kill five people. You are standing on a bridge above the track and, aware of the imminent disaster, you decide to jump on the track to block the trolley car. Although you will die, the five people will be saved.

Just before your leap, you realise that you are too light to stop the trolley. Next to you, a fat man is standing on the very edge of the bridge. He would certainly block the trolley, although he would undoubtedly die from the impact. A small nudge and he would fall right onto the track below. No one would ever know. Should you push him?
Again - have a think – what would you do?

Were your answers to the two scenarios different? If so, you’re not alone. In a survey conducted by the BBC, roughly 75% of respondents said they would flip the switch in the first scenario, versus 25% who said they would not. However, in the second scenario, these proportions were pretty much reversed: 27% said that they would push the fat man, while 73% said they would not.

Given that the moral dilemma is, at least on one level, exactly the same (either one person dies or 5 people die), comparing these two thought experiments raises some interesting questions about how we make moral choices and how (in)consistent our ethical decision-making processes are.

Questions to consider:

1) Most people understand that the issue in both thought experiments is the same (1 person dies or 5 people die), so why do you think that the response to the two scenarios varies so much?

2) Think about two of the ethical theories you’ve studied. If you apply the principles of these theories, which is the “correct” ethical response in each scenario? What might that tell you about your ethical theories and how useful they are in making ethical decisions?

3) When I’ve tried these thought experiments with GCSE and A level students, I find it’s very common for students to try to justify their decision by allocating some sort of blame which is not an explicit feature of the scenario (e.g. “it’s the people’s fault for being on the track”, “it’s the man’s fault for being fat”, “if the man was on the edge of the bridge, he was probably suicidal anyway”). What might these justifications tell us about the way we make ethical decisions?   

4) Do the issues raised by the two thought experiments have any implications for any real-life ethical issues?



  1. I don't know what I would do in such a situation, and I doubt anybody does know what they would do in such a situation, until they find themselves in it.

    1. Thanks for the comment Stephen.

      You’re right that one problem with thought experiments is that in practice what a person says they would do may bear no relation to the choice they would actually make in the real world. In any case, it would usually be impossible to prove how far a person’s response to a thought experiment corresponds to what they would actually do, since thought experiments often involve situations that would be impossible, unethical, or illegal to recreate in the real world.

      If you’d set up something like the Milgram experiment purely as a thought experiment, I’m sure that very few people would respond that they would give a hypothetical volunteer a potentially lethal electrical shock, but in the simulated reality of the Milgram experiment, in practice a high proportion of people did.

      On the other hand, perhaps the intention of though experiments like the runaway train isn’t to show what we would actually do, but to highlight some of the ethical issues the choice involves.

      Interestingly, there have been some attempts to run thought experiments in a virtual reality environment to make the dilemma involved more “real”.

  2. The most salient feature of the consistency of the trolley car thought experiment results is the implication that the bases of our ethical decisions are species-wide, rather than being inculcated into us. People know what is the right and wrong thing to do in each situation and mainly seem to agree about it, regardless of the seemingly illogical inconsistency between the two experiments. They do this without external guidance because there isn't any. This surely implies that, as a socially evolved species, we don't necessarily depend on such guidance.

    1. Thanks for the comment - it's a really interesting point. I do wonder though, how you draw the conclusion that the bases of our ethical decisions are *species* wide. The survey I refer to in the post was conducted by the BBC, so it's reasonable that the respondents were disproportionately British. As such, it would be hard to say whether respondents are influenced by the cultural and ethical baggage they bring with them or, as you suggest, by something more primal.

      I haven't seen any data that compares responses to the problem between different cultures, though I'd be very interested if you know of any. It would certainly be fascinating to know whether people in Japan or members of Brazil's Tupi tribe respond in the same way as people in the UK.