TORONTO -- Most people are able to feel empathy for a friend experiencing physical or emotional pain. But it's trickier when the person is a stranger -- and researchers suggest a major reason for that lack of fellow-feeling may be stress.

Perhaps surprisingly, the ability to share another's emotions -- or not -- is equally demonstrated by both mice and people, said Jeffrey Mogil, a neuroscientist at McGill University, whose team studied the link between stress and empathy in experiments on the laboratory animals and human volunteers.

"The really cool thing about this is that you get exactly the same findings in mice and people," Mogil said from Montreal.

Earlier studies had shown that mice and humans both have empathy for another's pain, especially when the individual in pain is familiar. Those studies also showed that stress levels rise in both rodents and people when they are in the presence of a stranger.

To look for a connection between stress and empathy, the researchers treated male mice with the drug metyrapone, which blocks the stress hormone cortisol, to see how they responded to pain in other mice.

They found the treated animals began reacting to unfamiliar mice in a manner normally reserved for cagemates -- they exhibited increased pain behaviours. But when the researchers put mice under stress, they showed less empathy when their fellow rodents exhibited signs of distress.

Researchers then assessed empathy in college students enrolled in the study using the "cold-pressor" test, in which the subject plunges one hand into ice water for 30 seconds, then rates the pain on a scale of no pain to the worst pain imaginable.

When tested alone, the students ranked the pain about midway on the scale. But they reported greater discomfort when paired with a friend also taking the test -- showing more pained facial expressions and often touching their own hands when witnessing another's suffering, the authors report Thursday in the journal Current Biology.

Mogil said the results are somewhat counterintuitive -- one would expect that being tested across from a friend would reduce the level of pain.

"But it doesn't make it better, it makes it worse," he said. "And the reason for that, we believe, is that your pain is being added onto by the empathy for your friend's pain. This extra bit of pain is actually coming from the contagion from the other person."

In contrast, when volunteers were paired with strangers, their pain ratings were the same as when tested alone. However, when subjects were given stress-blocking metyrapone, they reported increased pain from the ice water when tested with someone they didn't know -- just as they had with a friend.

The same reaction occurred when students, not given the drug, were paired with a stranger to play the game Rock Band together. The music video game involves simulating the performance of popular rock songs by playing with controllers modelled after musical instruments.

"We had them play a few Beatles songs together ... and 15 minutes after doing that we started the pain testing," Mogil said. "If they played it together, then they reacted just like friends would. They had the emotional contagion.

"Why? Well, the person's not a stranger anymore," he said, noting that lower cortisol levels in participants' saliva confirmed they were not experiencing stress.

"So we think what is preventing people from empathizing with each other on this very basic level is the fact that they're stressed because they find themselves in close quarters with someone they've never met before. So their empathy is unable to present itself."

All well and good, but what does this mean in the real world, outside the lab?

Mogil said many companies have been harnessing the phenomenon in a bid to promote cohesiveness among employees, for instance by gathering people from different departments together at a "retreat" to play get-to-know-you games.

"I think this is maybe the underlying reason for why team-building exercises exist because we've all sort of had this intuition that we're going to work better together if we empathize with each other, and the way to empathize better with people is to get to know them. There's less stress."

Tony Buchanan, a neuroscientist at St. Louis University who also studies the effects of stress, said the McGill findings suggest that finding ways to reduce stress could potentially increase interpersonal empathy.

"In Mogil's study, people showed this empathy response to their friends but not to other classmates, who are just like them presumably. They're just like them, but they don't know them," he said from St. Louis.

"The idea would be: how do we get that empathy between the dyads in this study to extend to people who are your neighbours, to people who live across a border from you, to people who are a different religion?"

Humans have been trying to figure that out for a long time, he said.

"This broadens the circle a little bit, but there's going to be barriers to how far that could go."