A trolley is hurtling down a track, and if nobody intervenes it will hit and kill five people. Psychologists use variations on this hypothetical situation to gauge people's gut reactions about morality. Here are three scenarios:

  • The driver could switch the train to another track, on which one man stands. Should the driver reroute the trolley?
  • Now suppose the trolley is driverless and you are a bystander. Should you hit a switch to divert the trolley so it hits the lone man?
  • You are standing above the tracks on a bridge. You could stop the trolley and save the five people by pushing a large man to his death in front of the trolley. Would you push him?

Most people say that the driver should reroute the train and that they would reroute the train with the switch but that they would not push the man to his death. This typical decision is associated with increased activity in the medial prefrontal cortex (green), which indicates a strong negative emotional reaction, as well as activity in the amygdala (red), which is involved in processing emotions and stressful events.

Some people do decide to push the man. This decision is associated with the following:

  • Increased activity in the dorsolateral prefrontal cortex (yellow), a center for cognitive control and reasoning.
  • Frontotemporal dementia. Patients with such damage to the frontal lobe (gray dashes) and temporal lobe (gray dots) show blunted emotions.
  • Ventromedial prefrontal lesions. People with this condition have less activity in the ventromedial prefrontal cortex (blue) and respond less emotionally overall.
  • High-functioning autism or Asperger's syndrome. Patients with these conditions often have impairments in emotional processing and social awareness.
  • Positive emotion induction. Healthy individuals shown a funny movie clip first are more likely to say they would push the man.
  • Prejudice. College-age study participants were more likely to say “yes” to pushing the man if he was described as an outsider (for instance, homeless, disabled, drug addicted or elderly) and if the five people to be saved were part of an in-crowd (American and young).

Can't decide? People who feel deeply divided on a moral issue show increased activity in the anterior cingulate cortex (purple), which is associated with internal conflict.