The core problem involves two thought experiments—call the first “Spur” and the second “Fat Man.”

In Spur, (see diagram one, below), an out-of-control trolley—or train—is hurtling towards five people on the track, who face certain death. You are nearby and, by turning a switch, could send the trolley onto a spur and save their lives. But one man is chained to the spur and would be killed if the trolley is diverted. Should you flick the switch?

In Fat Man (see diagram two), the same trolley is about to kill five people. This time, you are on a footbridge overlooking the track, next to a fat man. (The Fat Man is now sometimes described as a large gentleman. But fat or large, the fact of his corpulence is essential.) If you were to push him off the bridge onto the track his bulk would stop the trolley and save the lives of those five people—but kill him. Do you push him?

Study after study has shown that people will sacrifice the spur man but not the fat man. Yet in both cases, one person is killed to save five others. What, then, is the relevant ethical distinction between them? This question has spawned a thriving academic mini-industry, called trolleyology.