Would you kill someone if it would save the lives of five others? This classic thought experiment is known as the trolley problem, and is taking on growing importance as we train self-driving cars to take to the road. But the first real-life enactment of the problem in a lab – using mice – suggests we may have been approaching it wrong.

The trolley problem involves imagining that a runaway rail car is going to hit and kill five people – unless you pull a lever, diverting the car onto a …