Why would most people pull a lever to save five lives, but not push someone off a bridge?

Because, for most of us, how a death happens seems to matter as much as how many deaths there are. The trolley problem starts with a runaway trolley heading for five people on the tracks. You can pull a lever and divert it onto a side track, where it will kill just one person. In many surveys, about 90% of people say they would pull it.

Now change one detail. You are on a footbridge above the track, and the only way to stop the trolley is to push a heavy man off the bridge, killing him to save the five. The arithmetic hasn't moved, one life for five, yet a majority of people who approve of pulling the switch disapprove of pushing the man.

Philosophers disagree about what that shift means. A utilitarian says you are obliged to save the greater number. Others answer that by acting, you take part in a wrong and become partly responsible for a death that would otherwise be no one's fault. In 2001, psychologist Joshua Greene's brain scans added a clue: personal dilemmas like the footbridge light up regions linked to emotion more, while impersonal ones like the lever engage regions tied to controlled reasoning.

The puzzle has left the seminar room. Engineers of self-driving cars face trolley-style questions about what a car should hit when a crash can't be avoided.