Would you take one human life to save many? The obvious answer might seem to be “yes” —but what if your choice also meant you would be sacrificing your own child? Such dilemmas suggest that moral decision making has an emotional component, and now scientists have found the brain region responsible for generating these feelings.

Researchers studied patients with damage to their ventromedial prefrontal cortex, an area in the forebrain where social emotions such as compassion, guilt and shame arise. They asked the patients to respond to a variety of hypothetical moral dilemmas evoking emotional reactions of different strengths, then compared their responses with those of people whose forebrains were intact.

The subjects with damage showed a utilitarian approach in their answers, favoring the greater good regardless of the means required to achieve such ends. For example, many of them said they would smother their own baby to save a group of other people, whereas those with intact forebrains more often said they would not do so. In less emotionally fraught scenarios, all the study participants responded similarly. For instance, nearly everyone would choose to redirect deadly fumes from a room with three strangers to a room with one.

The findings show that our natural aversion to harming others emerges from two previously documented systems in the brain —one emotional and one rational. The emotional system pinpointed in this study triggers a fast, reflexive response; it provides a shortcut to what is right in situations requiring immediate action. The rational side aids us when deliberation and calculation are advantageous. Scientists do not yet understand how the two systems interact or how one supersedes the other when they dictate contradictory courses of action.

Moreover, people with damaged forebrains can still rely on their rational side to respond to moral dilemmas. “This study doesn’t mean that people who lack social emotions are dangerous,” says neuroscientist Michael Koenigs, then at the University of Iowa, a member of the research team. “They tend to show little empathy and guilt, but they are not killers.”

Researchers studied patients with damage to their ventromedial prefrontal cortex, an area in the forebrain where social emotions such as compassion, guilt and shame arise. They asked the patients to respond to a variety of hypothetical moral dilemmas evoking emotional reactions of different strengths, then compared their responses with those of people whose forebrains were intact.

The subjects with damage showed a utilitarian approach in their answers, favoring the greater good regardless of the means required to achieve such ends. For example, many of them said they would smother their own baby to save a group of other people, whereas those with intact forebrains more often said they would not do so. In less emotionally fraught scenarios, all the study participants responded similarly. For instance, nearly everyone would choose to redirect deadly fumes from a room with three strangers to a room with one.

The findings show that our natural aversion to harming others emerges from two previously documented systems in the brain —one emotional and one rational. The emotional system pinpointed in this study triggers a fast, reflexive response; it provides a shortcut to what is right in situations requiring immediate action. The rational side aids us when deliberation and calculation are advantageous. Scientists do not yet understand how the two systems interact or how one supersedes the other when they dictate contradictory courses of action.

Moreover, people with damaged forebrains can still rely on their rational side to respond to moral dilemmas. “This study doesn’t mean that people who lack social emotions are dangerous,” says neuroscientist Michael Koenigs, then at the University of Iowa, a member of the research team. “They tend to show little empathy and guilt, but they are not killers.”