The thought experiment—called the trolley problem—has over the past few years gotten enough attention to be approaching “needs no introduction” status. But it’s not quite there, so: An out-of-control trolley is headed for five people who will surely die unless you pull a lever that diverts it onto a track where it will instead kill one person. Would you—should you—pull the lever?
Now rewind the tape and suppose that you could avert the five deaths not by pulling a lever, but by pushing a very large man off a footbridge and onto the track, where his body would slow the train to a halt just in time to save everyone—except, of course, him. Would you do that? And, if you say yes the first time and no the second (as many people do), what’s your rationale? Isn’t it a one-for-five swap either way?
Greene’s inspiration was to do brain scans of people while they thought about the trolley problem. The results suggested that people who refused to save five lives by pushing an innocent bystander to his death were swayed by emotional parts of their brains, whereas people who chose the more utilitarian solution—keep as many people alive as possible—showed more activity in parts of the brain associated with logical thought.