Enemy soldiers stalk your village. You hide in a cellar with some neighbors and your baby, who starts to cry. Unless you keep his mouth covered, the soldiers will find you and kill everyone. Do you smother him to save yourself and your neighbors?

This moral dilemma is a notoriously agonizing staple of philosophy classes. A study shows that its difficulty may be caused by a battle in the brain.

Joshua D. Greene, a postdoctoral fellow at Princeton University, used fMRI scans to map subjects' brain activity. The "crying baby" question set the wheels turning in two zones: an area associated with emotion and areas linked to abstract reasoning and cognitive control. To referee the fight between incompatible emotional instincts ("Don't kill the baby") and cognitive responses ("Save more people"), an area thought to monitor conflict also kicked into gear.

The battle's outcome depended on just how hard cognitive brain areas fought. The more active these areas were, the more likely a person was to opt for killing the baby.

The fMRI scans reveal the neural equivalent of competing moral philosophies, says Greene. The social-emotional part of the brain pushes people to obey seemingly universal moral rules, such as an edict against murder, while the reasoning part pushes them toward a utilitarian goal: the greatest good for the greatest number of people.

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