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Why is stress so contagious?

It's amazing how quickly the best of moods can be soured by just a few minutes in the presence of a high-strung stress case. Their agitated movements, impatient glances and acrid responses can be like nails on a chalkboard. Even worse is when their anxiety and irritation gets under our skin, rapidly setting the tone for how we then feel and react to others. Instead of offering welcoming smiles, we become the ones giving dirty looks.

The dangers of secondhand stress and its contagious nature have long been considered obstacles to workplace productivity and general health and well-being. But how it happens has been less clear. Our thoughts, feelings and actions all arise through different patterns of brain activity. So how is it that one person's bad day can hijack another person's brain?

One possibility concerns a recently discovered class of brain cells called mirror neurons. Originally discovered in monkeys, these cells are thought to "reflect" the actions and feelings of others. The cells in the mirror-neuron system that fire when a monkey reaches out to grab a piece of food, for example, also fire when the monkey sees someone else reaching out to grab it.

There is growing evidence of a similar mirror system in the human brain. Related work has focused on our emotional responses. Why do we cringe when we see someone else get hurt? Because we mentally simulate the event as if it were happening to us.

Indeed, human neuroimaging studies have shown that the pain we feel for others when they get hurt activates many of the same regions that are activated when we get hurt ourselves. These include the dorsal anterior cingulate cortex near the top and centre of the frontal lobe and anterior parts of the insular cortices folded deep on each side of the brain between the frontal and temporal lobes.

Such specialized brain cells and neural circuits therefore seem to be important for experiencing empathy. They also help in understanding the intentions and emotional expressions of others. But mentally reinstating the emotions of others may be a serious mood-killer if they are stressed and anxious.

Their sweat might also be a problem. It seems that stress leads to the release of pheromones during perspiration. As with other pheromones, these chemical secretions can unconsciously impact brain activity and behaviour when inhaled by others. Indeed brain scans have shown that a whiff of stress-produced sweat is enough to increase activation within the amygdala, an emotion-related region of the brain nestled deep within each hemisphere.

The amygdala is also activated during the direct experience of anxiety and is thought to help us respond to potential dangers. Scalp-based measures of electrical activity suggest that, compared to exercise-produced sweat, inhaling a stressed-person's sweat can cause the brain to increasingly treat others as potential threats, even if their facial expressions suggest they are actually harmless. This may explain why we snap at those we might otherwise treat with greater respect when we experience secondhand stress.

Understanding how easily stress can pass from one person to another underscores the importance of reducing our own stress so that we are not spreading it to others. Research has shown physical exercise to be particularly effective at reducing stress-related anxiety and depression. The enhanced emotional control provided by meditation may also inoculate you against secondhand stress.

And we can benefit from the contagious spread of positive emotions, too. Smiling and laughter, for example, are well known for their infectious nature. Acts of kindness, concern and patience for those having a bad day can likewise help to stem an epidemic of stress.

Mark Fenske, co-author of The Winner's Brain: 8 Strategies Great Minds Use to Achieve Success , is an associate professor in neuroscience at the University of Guelph.

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