The Trouble With Trolleys

Listening the other day to BBC Radio 4, the UK's national talk radio station, I caught the end of one of those strange shows that attempt to prove that being intellectual can be cool. (As usual, it failed dramatically to do so.) Called The Philosopher's Arms, it is supposed to be set in a pub and features audience interaction that involves clinking cutlery on glasses. Really. I usually change channel as soon as I come across it.

This time, though, the subject was trolley ethics, which I find fascinating, so I gritted my teeth and kept listening. My hope was that a big problem with the best-known example of this philosophical discipline would be explored. Sadly, though, it was left undiscussed.

There are plenty of variants on trolley ethics, but they all involve thought experiments to explore a human dilemma. As the name suggests, these experiments feature a trolley. (In the UK we'd call it a tram.)

The idea is that a trolley is out of control, running along a downhill track. You are sitting in a control room where you can press a button to change the direction of a switch (points) in the track. If you take no action, the trolley will smash into and kill five people. But if you press the button, you can divert the trolley and save those five people. Unfortunately this will send the trolley down a track on which one person is standing -- he or she will be killed instead. Would you press the button?

The majority of people asked to make this decision say yes. Would you? Admittedly, you would be responsible for that one person's death -- but if you took no action, you are responsible for five deaths, as you could have saved their lives.

Then you are presented with an alternative. Imagine you are on a bridge that runs over the trolley track. A runaway trolley is about to shoot under the bridge and kill five people. (This is a very badly run trolley line.) The only way to stop this from happening is to push a very heavy person, who is standing next to you, off the bridge. Because the person is so heavy, this will stop the trolley, but it will inevitably kill the heavy person. What would you do?

When the thought experiment is undertaken, most say no, they couldn't take this action, even though it is logically identical to pressing the button in the first example. Once more, if you did it, you would be sacrificing one life to save five. The change in response is used by philosophers and psychologists to explore the way that emotional connection changes reasoning. In the first case you perform a cold, dispassionate button push. In the second you are directly killing another human being, with all the emotional baggage that carries. Some suggest it is similar to the distinction between pulling the trigger on a sniper's rifle and killing someone with your bare hands.

I understand what the academics are attempting to do, but I don't believe that the experiment as stated proves the point at all. (It may be there are other controls that aren't mentioned when the experiment is carried out for real, but they weren't used in the radio show.) Between the two cases there is not just a difference in emotional connection but a cold logical difference too.

In the first case I can pretty well guarantee that the outcome will be as described. Assuming the controls work properly, pushing that button will save five lives while sacrificing one other. But in the second experiment there are two chunky flaws. First, I have to be able to push an extremely heavy person off a bridge. Would I be strong enough? OK, you can get around this by saying the big person happens to be perched on the parapet, standing on one leg in an attempt to take a dangerously idiotic selfie.

The second problem is harder to get around. The heavy person's body has to stop the trolley. Simple physics tells me this is extremely unlikely. Admittedly, we can extend the thought experiment by saying that the heavy person falls on a control that moves a switch to divert the trolley, and only a very heavy person's body would do this. But even then there is no way that the outcome can be guaranteed. Our big guy might not fall in the right place to save the day.

The result is that the choice is not just between a low emotional connection and a high emotional connection, as is usually stated. It is the difference between killing someone with a certainty of saving others and killing someone where the associated benefit is nothing more than a long shot. And that very much changes the decision parameters.

I have no doubt that those running the experiment would reassure test subjects that the heavy person would definitely stop the trolley -- but such reassurance can't stop the part of the brain that weighs up the odds from saying "Nah, it's not going to work. I'm going to kill this guy, and the trolley will still plough into the others." In reality, test subjects would always be in doubt about the second solution.

For me, that makes the whole experiment fatally (if you'll pardon the expression) flawed.

Brian Clegg is a popular science author, with titles including Dice World, Final Frontier and The Quantum Age. For more information visit