Consider the following scenario: someone you know has AIDS and plans to infect others, some of whom will die. Your only options are to let it happen or to kill the person.
Do you pull the trigger?
Most people waver or say they could not, even if they agree that in theory they should. But according to a new study in the journal Nature, subjects with damage to a part of the frontal lobe make a less personal calculation.
The logical choice, they say, is to sacrifice one life to save many.
Conducted by researchers at the University of Southern California, Harvard University, Caltech and the University of Iowa, the study shows that emotion plays an important role in scenarios that pose a moral dilemma.
If certain emotions are blocked, we make decisions that - right or wrong - seem unnaturally cold.
The scenarios in the study are extreme, but the core dilemma is not: should one confront a co-worker, challenge a neighbor, or scold a loved one in the interest of the greater good?
A total of 30 subjects of both genders faced a set of scenarios pitting immediate harm to one person against future certain harm to many. Six had damage to the ventromedial prefrontal cortex (VMPC), a small region behind the forehead, while 12 had brain damage elsewhere, and another 12 had no damage.
The subjects with VMPC damage stood out in their stated willingness to harm an individual - a prospect that usually generates strong aversion.
"Because of their brain damage, they have abnormal social emotions in real life. They lack empathy and compassion," said Ralph Adolphs, Bren Professor of Psychology and Neuroscience at Caltech.
"In those circumstances most people without this specific brain damage will be torn. But these particular subjects seem to lack that conflict," said co-senior author Antonio Damasio, director of the Brain and Creativity Institute and holder of the David Dornsife Chair in Neuroscience at USC.
"Our work provides the first causal account of the role of emotions in moral judgments," said co-senior author Marc Hauser, professor of psychology at Harvard and Harvard College Professor.
But, Hauser added, not all moral reasoning depends so strongly on emotion.
"What is absolutely astonishing about our results is how selective the deficit is," he said. "Damage to the frontal lobe leaves intact a suite of moral problem solving abilities, but damages judgments in which an aversive action is put into direct conflict with a strong utilitarian outcome."
It is the feeling of aversion that normally blocks humans from harming each other. Damasio described it as "a combination of rejection of the act, but combined with the social emotion of compassion for that particular person."
"The question is, are the social emotions necessary to make these moral judgments," Adolphs asked.
The study's answer will inform a classic philosophical debate on whether humans make moral judgments based on norms and societal rules, or based on their emotions.
The study holds another implication for philosophy.
By showing that humans are neurologically unfit for strict utilitarian thinking, the study suggests that neuroscience may be able to test different philosophies for compatibility with human nature.
The Nature study expands on work on emotion and decision-making that Damasio began in the early 1990s and that caught the public eye in his first book, Descartes' Error.
Marc Hauser, whose behavioral work in animals has attempted to identify precursors to moral behavior, then teamed up with Damasio's group to extend those observations.
Other authors on the study were Fiery Cushman and Liane Young of Harvard, and Michael Koenigs and Daniel Tranel of the University of Iowa.
Funding for the research came from the National Institutes of Health, the National Science Foundation, the Gordon and Betty Moore Foundation, and the Guggenheim Foundation.