That look151;it is catching!

Emotional contagion occurs in a matter of milliseconds, says Elaine Hatfield, a professor of psychology at the University of Hawaii and co-author of 8220;Emotional Contagion8221;. If you8217;re the receiver, you may not even know what happened, just that you feel differently after the encounter.

The phenomenon depends on a primal instinct: during conversation, humans unconsciously tend to mimic and synchronize the other person8217;s facial expressions, posture, body language, says John T Cacioppo, professor of psychology and director of the Center for Cognitive and Social Neuroscience at the University of Chicago.

When it comes to this monkey-see, monkey-do dynamic, 8220;the more expressive and sincere someone is, the more likely you are to see that expression and mimic it unknowingly,8221; Cacioppo says.

But there may be another mode of transmission: In the course of conversation, people have a tendency to match the emotional tone of the other person, according to research presented at the Midwestern Psychological Association8217;s annual meeting in May.

8220;Communication requires matching of specific words and contents so people can understand each other,8221; says the study8217;s co-author Frank Bernieri, associate professor of psychology at Oregon State University in Corvallis.

For the vast majority of people, you want to know what other people are thinking, to be in sync with them, to have sympathy and empathy.

Not only can this help people connect on a very basic emotional level, but it has practical applications too, including survival value, benefits in the face of conflict such as a marital spat or competition such as a sporting event. And it may be useful in work settings.

The degree to which people become emotionally in sync depends partly on their level of intimacy and emotional investment in the relationship.

Not surprisingly, people living under the same roof are especially likely to catch each other8217;s moods8212;spouses and room-mates, found researchers at Arlene R. Gordon Research Institute in New York.

While some people are more prone to infecting others with their moods, others are more likely to become engulfed by people8217;s emotions.

People who are more expressive, may be more likely to spread their emotions. On the other hand, people who respond strongly internally to emotional events, may be more susceptible to catching other people8217;s moods, Cacioppo says.

With any luck, people catch the positive emotions8212;a colleague8217;s enthusiasm at work, a friend8217;s excitement over an athletic event8212;and miss the negative ones. Some psychologists suspect, however, that negative emotions may be more infectious. 8220;If someone is sharing negative emotions in a self-disclosing, personal realm, you have to be empathic and acknowledge it,8221; Bernieri says, which makes the emotion more likely to spread.

8211;Stacey Colino