Your Brain on Justice

Here's a thought experiment every freshman Ethics student confronts.

A runaway trolley is hurtling towards a group of children loitering on the tracks. If nothing is done, they'll soon be crushed. But something can be done: next to you is a switch that, if thrown, will divert the trolley onto a sidetrack. The only thing is, on the sidetrack is another child-who'll be killed by the diverted trolley. You scream to the children, but to no avail. The question is this: What would be the right thing to do here? Do nothing or throw the switch?

Once you have your answer, consider this variation.

A runaway trolley is again hurtling towards a group of children on the tracks. If nothing is done, they'll soon be crushed. But something can be done: next to you on a footbridge above the tracks is a very large man who, if pushed onto the tracks, will halt the trolley before it reaches the children. The trolley, however, will surely kill the man. What would be the right thing to do here? Do nothing or push the man?

Despite identical consequences (five saved, one killed), most respondents draw a distinction. It's OK to throw the switch, they say, but not OK to push the man. So what's going on here?

Moral philosophers tend to offer something along these lines: it's OK to throw the switch because you're not using the child's death as a means to an end. His death is merely a foreseeable side-effect. However, pushing the man onto the tracks is not OK, since you would be using the man's death as a means to an end. And it's always unjust to harm others as a means to an end.

Harvard psychologist Joshua Greene doubts the answer lies among eternal moral truths. He believes the answer lies in the brain. When Greene's subjects considered these scenarios, he recorded their brain activity, and what he found were patterns suggesting that it's not justice that's driving our judgments about justice, but emotion. In the Footbridge case, the areas associated with emotion (i.e. the posterior cingulate cortex, the medial prefrontal cortex, and the amygdala) were active. In the original Trolley case, the areas associated with cognition (the dorsolateral prefrontal cortex and inferior parietal lobe) were active.

According to Greene, the thought of pushing a man to his death-despite good intentions-triggers powerful negative feelings, feelings that almost certainly originated during our evolutionary past. And it's these feelings that deliver the negative judgment in Footbridge. Citing the demands of justice is just the brain's way of making sense of these formless stirrings. Because such feelings are absent in the Trolley, we're willing to "go with the numbers:" it's OK to throw the switch. Greene contends that this "up close and personal" theory of moral judgment makes sense of a range of cognitive psychology data.

If Greene is correct, the message could upend centuries of moral thought. For what we have is not a justification for our judgments about justice, but merely an explanation. There's a reason we judge that pushing the man off the footbridge is wrong-but that reason has nothing to do with wrongness. The reason has to do with primitive emotions operating beneath our conscious deliberations. We have to make sense of these feelings (to be sure), and so we grasp at philosophical straws: justice, using others, foreseeable side-effects. But in the end, it's the emotions that wag the moral dog.