In a 2002 study, a Yale University researcher found that emotions don't just hop from one person to another; they also influence group dynamics. Sigal G. Barsade, PhD, now a professor of management at the Wharton School at the University of Pennsylvania, separated 94 business students into small groups, each with the same hypothetical task of allocating employee bonuses. Barsade secretly planted one student in each group to act out a different emotion: enthusiasm, hostility, serenity, or depression. When the infiltrator was enthusiastic, he smiled often, looked intently into people's eyes, and spoke rapidly. When he feigned depression, he spoke slowly, avoided eye contact, and slouched in his seat.

Barsade measured participants' moods before and after the exercise and found that students who caught the actor's positive emotions were perceived by others and by themselves as more competent and cooperative. The positive groups also believed they were more collegial than those in the bad-mood groups. But when Barsade asked the students what influenced their performance, they attributed it to their skills. "People don't realize they are being influenced by others' emotions," she says.

Totterdell noted similar results when he studied four teams of professional cricket players: The overall good mood of the team lifted spirits of individual players, and athletes on happier teams played better. Unhappy moods transferred less readily, which Totterdell suspects is because gloomy players tended to withdraw emotionally from the team. He also found that players were much more likely to catch one another's emotions when they were involved in a shared effort like fielding than an individual one like batting.

This effect is magnified in marriage—a relationship largely founded on emotions. A glum spouse can torpedo the spirits of his wife, Totterdell says. A study of older couples at the Arlene R. Gordon Research Institute in New York bears this out: One spouse's depression predicted the onset of the other's. And in a study at the Medical University of South Carolina in Charleston, researchers found that the stress borne by medical students resulted in a depression that they transferred to their partners.

Interestingly, negative emotions are usually more infectious than positive ones. Despite the cricket-player study, humans usually react more strongly to pain, fear, sadness, and disgust than to joy and serenity, says John T. Cacioppo, PhD, professor of psychology at the University of Chicago and coauthor, along with Hatfield and Richard L. Rapson, of Emotional Contagion. One of the functions of sadness, he says, is to solicit help from others. Pain, fear, and disgust are usually linked directly to survival instinct. If, in the presence of predators, prehistoric man had reacted more strongly to food than to danger, he would have been less likely to survive and reproduce. "When both good and bad are very strong, the bad trumps," Cacioppo says. In fact, when he recorded electrical activity in the brain, he found that negative emotions elicited a stronger reaction.

Some people are also particularly vulnerable to catching emotion; others excel at emitting it. Expressive, dramatic personalities send stronger signals, while, not surprisingly, attentive, empathetic observers are more likely to pick up on someone else's emotional display. One new clue to susceptibility comes in the form of yawns. In a 2003 study at the State University of New York at Albany, lead author and biopsychologist Steven Platek, PhD, found that those who caught other people's yawns tended to be more empathetic than those who didn't, although it's not clear why. Research also suggests that women are much better than men at reading, and thus catching, others' emotions. As women, Hatfield says, "we're trained from a young age to be sensitive to what others are feeling. Men usually aren't, but that's changing."

Being an attentive woman doesn't automatically make you susceptible to every passing storm. To determine whether you're a mood catcher, watch for rapid changes in your feelings. If you were in a fine mood but suddenly tense up when you join your husband, that's a clue: He may have brought his irritability with him. Or if every time you talk to a certain colleague you feel an unfamiliar awkwardness, it could be your coworker having the tough time. Once you recognize that you've caught someone else's mood, try distancing yourself. If your husband's doldrums are tamping your joy, look for a chance to retreat. "Be kind and loving for an hour, then take a break," says Hatfield. "Read, go to the movies, or take off with cheerful people." Preempt grumpiness brought on by visits to your in-laws by booking a hotel room. Counter-contagion is another helpful trick: Try offsetting your mate's moodiness with smiles. If his bad mood is chronic, though, Barsade suggests telling him exactly how he makes you feel. You can't cure his depression, but sometimes just knowing he's affecting you can help motivate him to break the pattern. "He may not even be aware of the emotion he's transferring," she says.