Doctors learn to shut down the part of the brain that empathises with the pain their patients suffer during treatment, so they can avoid becoming distressed by conducting unpleasant medical procedures.
Brain scans have revealed that doctors show a different pattern of neural activity to other people when watching videos of painful therapies.
The findings indicate that medical training and experience allows them to override the normal human response to seeing others' suffering, which would otherwise hinder their ability to treat patients.
The research, led by Jean Decety, Professor of Psychology and Psychiatry at the University of Chicago, and Yawei Chang of the Taiwan Institute of Neuroscience, could also shed light on humanity's capacity for cruelty.
While the study did not test psychopaths, torturers or war criminals, it is possible that such people also have a genetic or learnt ability to suppress brain circuits that would normally lead them to empathise with their victims.
It is well known from previous research, some led by Professor Decety, that the parts of the brain that register pain can be activated vicariously by seeing another person suffering. While viewers do not necessarily then feel pain themselves, it often leads to a "flinch" or "panic" response.
This can often be beneficial, and may have evolved as a way of avoiding danger and aggressive actions that might put the viewer at risk from retaliation. In a medical context, however, it can be counterproductive.
Many therapies require doctors to conduct examinations, perform operations or administer drugs that will be uncomfortable, painful or distressing to their patients. A normal reaction to inflicting this pain would limit their capacity to treat people.
The new research suggests that they have learnt to control this, to allow them to do their jobs more effectively. "They have learnt through their training and practice to keep a detached perspective," Professor Decety said.
"Without such a mechanism, performing their practice could be overwhelming or distressing and, as a consequence, impair their ability to be of assistance for their patients."
In the study, published in the journal Current Biology, the scientists recruited 14 doctors, half male and half female, with an average age of 35. They then assembled a control group of people without medical training, who were matched for age, sex and socio-economic status.
All then had their brains scanned using functional magnetic resonance imaging (fMRI) while watching two videos. In the first, the participants saw patients having acupuncture needles in their mouth, hands and feet, a potentially painful procedure. In the second, they watched the same parts of the body being prodded with cotton buds.
During the pain videos, the control group showed higher activity in parts of the brain called the anterior insula, the periaqueductal grey and the anterior cingulate cortex. These are all known to be involved in the sympathetic pain response. The circuit did not respond during the cotton bud video.
Such raised activity was completely absent when doctors watched both videos. Instead, when doctors watched the pain video they showed increased activity in the medial and superior parts of the prefrontal cortex, regions that are known to be involved in controlling emotions.
Both groups were also asked to rate the level of pain felt by the subjects in the acupuncture video, on a scale of one to ten. The control group recorded an average of seven points, against three for the doctors, indicating that doctors tend to think of such procedures as less painful than the public at large.
The scientists said the phenomenon appears to be a learnt response to an instinctive brain reaction that would normally impair medical practice.
"It would not be adaptive if this automatic sharing mechanism for pain was not modulated by cognitive control," they wrote. "Think, for instance, of the situations that surgeons, dentists, and nurses face in their everyday professional practices.
"Without some regulatory mechanism, it is very likely that medical practioners would experience personal distress and anxiety that would interfere with their ability to heal."