It can happen when you see someone else doing it. And sometimes, just reading about it is enough to trigger the response. You may be doing it right now. Indeed, yawning is very contagious.

And Gordon G. Gallup, Jr., professor of psychology at State University of New York at Albany, said no one really knows why.

"Nobody's been able to identify the physiological trigger for contagious yawning or even yawning per se being contagious," he said.

Gallup contributed to a research study published this year entitled, "Contagious yawning: the role of self-awareness and mental state attribution." In the study, Gallup and his colleagues found that contagious yawning is linked to someone's ability to empathize with another person.

"When we tested a large number of college students, and they were showed videos of people yawning or not, we found that just under 50 percent showed contagious yawning," he said.

There were no differences in contagious yawning as far as students viewing people of their own or the opposite sex yawning, Gallup said. Basically, if someone was going to yawn while watching the video, it did not depend on the gender of the student or the person in the video.

In another part of the study, people were shown pictures of their face and the faces of others on a monitor. "People that were shown their own face and identified their own face faster than other people were more likely to yawn contagiously," Gallup said.

The researchers also administered mental state attribution tests, which measure a person's ability to make inferences about what others are thinking.

In the tests, Gallup said, one might have to infer that others want to do something or have certain intentions. "People who show contagious yawning are better at making inferences at what people want," he said.

Schizotypal personality questionnaires were also used to determine if a person was schizophrenic. People that scored low on the scizotypal personality tests [therefore having less schizophrenic traits] were more likely to show contagious yawning than those who had higher scores, Gallup said,

Based on the results of the tests, Gallup and the other researchers associated contagious yawning with having the ability to empathize and conceive of one's self.

"There is growing evidence that schizophrenics show deficient self-conception," he said. "They can't distinguish between behaviors that are self-initiated and behaviors that are not."

For example, Gallup said, there is evidence that when schizophrenics hear voices, a common symptom of schizophrenia, the voices are usually their own, and not those of others. "If you tell [a schizophrenic] to open their mouth as wide as they can [when they hear the voices], the voices stop."

Also, because of the self-processing deficit, if a schizophrenic sees his or her own hand in addition to someone else's on a video monitor, the schizophrenic cannot distinguish his or her own hand from the other. "Normally, you would move your hand around in a certain direction and you could then say which was yours, but [schizophrenics] can't say which is theirs," he said.

Other evidence that supports the correlation between contagious yawning and self-identification can be found in studies of children, Gallup said.

"Little kids don't begin to show contagious yawning until two years old," he said. "The ability to conceive of their self also begins at two. That is when they start to recognize themselves in mirrors."

A popular myth about yawning is that it occurs because there is not enough oxygen in the bloodstream, but Gallup disagreed.

Robert Provine, professor of psychology at University of Baltimore, Maryland County and author of Laughter: A Scientific Investigation, said it has been proved that yawning is not associated with a lack of oxygen. "In a study, it was found that breathing enhanced levels of carbon dioxide doesn't make you yawn more," he said.

The reason why people yawn in the first place is unknown, Gallup said.

"Yawning is a widespread trait," he said. "Many, if not all, mammals show yawning behavior. It is interesting in its own rite in functional significance, why we do it, or what the pay off is when we yawn."

Provine said yawning occurs naturally. "You don't have to learn how to yawn and you don't choose to yawn," he said. "It just happens."

Yawning is associated with changes of behavior, he said.

"It serves to bond people together in groups," Provine said. "You see someone yawn, so you yawn, then someone else yawns, and so on. Basically you have a group of synchronized yawning. The synchronizing of yawning may help us perform those changes in life."

Being bored and even anxious triggers yawning, Provine said. "You yawn more when bored and before bedtime," he said. "But you probably yawn the most after you wake up."

Penn State students find they yawn for various reasons and at different times of the day.

Jennifer Maximos (sophomore-biology) finds that she yawns contagiously. "I yawn when I'm most tired and when people around me are yawning," she said.

Sean Galvin (sophomore-criminal psychology) said seeing other people yawn does not cause him to yawn. "I yawn most right before I go to bed," he said.

While Gallup said he and his colleagues learned a lot from the study, he is not currently studying contagious yawning.

"We're doing a lot of work on self-awareness, mental state attribution and premorbid schizophrenic tendencies," he said. "Contagious yawning was inspired by that, but we're not pursuing it per se."

Provine is currently studying contagious yawning but would not reveal the details of his research.

"We're interested in the role of spontaneous and contagious behavior," he said. "We're studying the ancient primitive aspects of behavior."

Psychologists should do more research on the origin of behaviors, Provine said.

"People studying human behavior are interested about things occurring in this lifetime but don't look at ancient and distinctive behaviors."