If the person next to you on the bus yawns, chances are you know that you'll soon be yawning, too. What you may not realize is that you may also be mimicking your seatmate's grimaces, smiles and frowns.

Emotions are contagious. And interestingly, much of this process may be subconscious — though not any less important for how people interact.

Studies going back decades have found that when people observe someone's facial expression, electrodes on their own facial muscles record muscle activity, as if they were making a tiny version of the same expression. Typically, these micro-moments of mimicry are invisible to the naked eye.

Other studies find that people similarly copy other people's posture and vocal tics. And they seem to be quite good at it. One 2010 study, published in the journal Attention, Perception & Psychophysics, found that when people lipread, they sound like the person they're lipreading from, even though they've never heard that person's voice.

But the copycatting doesn't end with muscular movements. Research finds that emotions propagate from person to person, too. The theory, according to University of Hawaii psychologist Elaine Hatfield, is that emotions don't always start in the mind. They also travel from body to brain. So if your coworker's posture is slouched and the corners of his mouth are turned down, you'll subtly mimic his posture. Then, your brain will take cues from your own body language, and you'll start to feel like you look: Totally bummed out.

Emotional matters

Some scientists explain the neurological roots of emotional contagion through the concept of mirror neurons. These much-ballyhooed brain cells are thought to fire both when people perform an action and when they observe someone else performing that action. Some researchers suspect these mirror neurons are at the root of the human ability to put ourselves in others' shoes, though it's likely that their role is nuanced and complex. (Researchers can investigate mirror neurons in monkeys by inserting electrodes next to a single neuron in the brain, but evidence for the mirror system in humans is based on cruder brain imaging techniques. As a result, there is a lot of academic debate about what mirror neurons do in humans or if the system even exists.)

Whether mirror neurons are involved in emotional contagion or not, the catching nature of emotions has caught the attention of psychologists studying everything from workplace culture to the spread of terrorism. What they've found suggests that emotional contagion makes a big difference in daily life.

Sigal Barsade, a professor of management at the University of Pennsylvania's Wharton School, launched a study of emotional contagion more than a decade ago in which she asked undergraduates to work together in groups to assign salary bonuses to imaginary employees. Unbeknownst to the participants, one member of each group was an actor. The actor pretended to be either in a good mood or a bad mood, at varying levels of energy. In a good, high-energy mood, for example, he was warm, alert and enthusiastic. In a good, low-energy mood, he gave off a sense of serenity and calm.

In his high-energy bad mood, the actor came across as pessimistic and actively hostile. In his low-energy bad mood, he was slouched and sluggish. Barsade expected that bad moods would spread easier than good moods, given the fact that humans tend to pay more attention to negative stimuli than to positive stimuli.

That's not what happened. In the study, published in December 2002 in the journal Administrative Science Quarterly, both pleasant and unpleasant moods spread at similar levels. There were suggestions that level of energy might have mattered. When emotions were intense, negatively spread more easily than positivity. But the emotion that spread best of all was what Barsade calls the California Condition: calm serenity.

Most importantly, the emotions at play mattered for how much the groups cooperated and how effective they were at their task. This real-world applicability is something that has come up again and again in research. In another study, published in 2011 in the Journal of Personality and Social Psychology, Barsade and her team set out to learn whether it's better to start a tough negotiation in a good mood and then get angry, or whether it's better to start out as a tough guy and then soften up. They had pairs of people negotiate and found that the winning strategy was to start happy and get mad.

This sort of emotional Jekyll-and-Hyde trick might seem counterintuitive, but anger is known to be a winning strategy in short-term negotiations, Barsade said. (Long-term negotiations are another story.) Starting out happy let people keep all of the monetary benefits of an angry negotiating style, but it also kept their negotiating partner from disliking them. Part of the reason? The initial happy mood was contagious, and its spread kept the second partner from becoming as upset with the anger as they otherwise would have been.

Questions about contagion

These days, of course, many work conversations take place over email or on Slack or other messaging services. But emotions spread through electronic media, too. Contagion is powerful, said the University of Hawaii's Hatfield, and people tend to mimic others' word choice and emotional tone online. A famous study of Facebook users found that when people see more positive posts in their feed, they made more positive posts. When they saw more negative posts, their posts got gloomier, too.

Researchers have looked at the transmission of emotions through email, Barsade said, and they've found that the emotions that land in your inbox can affect you. The problem is that people are notoriously bad at judging the emotions in an email, she said, so misinterpretations abound. In other words, it's quite possible that a sender could infect you with an emotion they were never actually feeling.

Other workplaces conundrums need more research. What if a Debbie Downer and a Chipper Charlie share a cubicle? Whose mood wins out? No one knows, Barsade said. Attention is clearly key, because the more we pay attention to a person, the more we catch their emotions. But it's not clear whether negative emotions or positive emotions ultimately carry the day.

Nor is it clear how people consciously grapple with their unconscious tendency to feel other people's feelings. Emotional contagion is a largely automatic process, but that doesn't mean the conscious cognitive processes can't swoop in and make adjustments. Along with Boston University researcher Kristin Smith-Crowe and the Wharton School's Jaime Potter, Barsade is studying how people respond when their enemies display an emotion. Preliminary results suggest that sports fans initially get a millisecond's happiness from seeing the opposing team's fans cheer. But very quickly,. "the brain comes in and says, 'wait, wait, wait,'" Barsade said. At that point, their emotions turn negative as they realize that happiness for the other guys likely means bad news for their own side.

It's also important to remember that people don't wear their every emotion on their sleeves. Emotional displays are a communication system, said Guillaume Dezecache, a researcher in the department of comparative cognition at the Université de Neuchatel. Ignoring the flexibility with which humans display and respond to emotions hinders research into the factors that affect those responses, he said.

Meanwhile, researchers like Hatfield are increasingly interested in studying how emotions are transmitted not just between two people but also across large groups, hoping to explain a variety of social phenomena, for example, how terrorist groups recruit followers.

"Given the current climate of hatred, people have started to examine why appeals to hate are so powerful," Hatfield said. "An intellectual appeal to tolerance and understanding has a hard time competing with messages that stir up emotions."