They strongly favored flipping the switch, just as group of people without injuries did. A third group, with brain damage that did not affect the ventromedial cortex, made the same decision.

All three groups also strongly rejected doing harm to others in situations that were not a matter of trading one certain death for another. They would not send a daughter to work in the pornography industry to fend off crushing poverty, or kill an infant they felt they could not care for.

But a large difference in the participants’ decisions emerged when there was no switch to flip — when they had to choose between taking direct action to kill or harm someone (pushing him in front of the runaway boxcar, for example) and serving a greater good.

Those with ventromedial injuries were about twice as likely as the other participants to say they would push someone in front of the train (if that was the only option), or to poison someone with AIDS who was bent on infecting others, or suffocate a baby whose crying would reveal to enemy soldiers where the subject and family and friends were hiding.

“The difference was very clear, for all of the ventromedial patients,” said Dr. Michael Koenigs, a neuroscientist at the National Institutes of Health who led the study while at the University of Iowa. After repeatedly endorsing killing in these high-conflict situations, he added, one patient told him, “Jeez, I’ve turned into a killer.”

The other authors were Dr. Daniel Tranel of Iowa and neuroscientists from Harvard, the University of Southern California and the California Institute of Technology.

The ventromedial area is a primitive part of the cortex that appears to have evolved to help humans and other mammals navigate social interactions. The area has connections to deeper, unconscious regions like the brain stem, which transmit physical sensations of attraction or discomfort; and the amygdala, a gumdrop of neural tissue that registers threats, social and otherwise. The ventromedial area integrates these signals with others from the cortex, including emotional memories, to help generate familiar social reactions.