According to these scientists:
Researchers from the State University of New York in Albany tested people to find out why some are susceptible to contagious yawning and deduced that self-aware or empathetic people are more likely to catch yawns, according to the news service.
"Identifying with another's state of mind while they yawn may trigger an unconscious impersonation....The findings also explain why schizophrenics, who have particular difficulty in doing this, rarely catch yawns," Nature News Service reported.
Broughton said contagious yawning may be a social phenomenon, allowing groups of humans to coordinate their times of sleep.
Ronald Baenninger, a professor in the psychology department at Temple University in Pennsylvania, has studied yawning and said that the contagiousness is the part we know the least about.
One hypothesis is that the phenomenon came about when ancestral humans lived in troops, and it was important for them to wake up at the same time. Yawning may have been a way for them to communicate the level of alertness among different group members, Baenninger said.
Today, contagious yawning is just the result of evolution, an instinctive action, Baenninger said.
Though other creatures yawn - including dogs, cats and even snakes - contagious yawning is a "purely human occurrence," Broughton said. It begins around age 2.
Baenninger and his students conducted a study to see if the contagion works between species. The students went to the zoo to observe whether humans would yawn when the animals did. A few people yawned in response to a lion's yawn, but the lion never replicated the humans' behavior, Baenninger said.
There is some evidence that when one ape yawns, others will too, the professor said.
Broughton said it's not clear what causes yawns. He defined yawning as "slow, involuntary gaping movements of the mouth," and said the word is derived from the Old English word ganien, which means "to gape."
Spontaneous yawning begins in the embryonic stage, so it is not learned behavior, Broughton said.
The idea of a fetus yawning while in the womb goes along with the notion of the action being a way to prepare for something, Baenninger said.
Yawning appears to be associated with sleepiness, though the idea that it has to do with boredom is not necessarily true, Broughton said. It's an unconscious effort to keep your alertness level elevated, Baenninger said.
"I'm a professor. All professors are used to having students yawn at them," Baenninger said. But the yawns are an attempt to stay awake, rather than an affront, he said.
A few notions about why we yawn have been debunked: "Because breathing takes in oxygen and removes carbon dioxide, theories in the past about why we yawn centered on the assumption that it was a reflex in response to low oxygen or high carbon dioxide levels," writes Dr. Robert H. Shmerling, an associate physician at Beth Israel Deaconess Medical Center and an associate professor at Harvard Medical School.
"This theory lost favor after a study in 1987, in which volunteers subjected to high oxygen levels did not yawn less, and after high carbon dioxide exposure did not yawn more," Shmerling said.
In an article, he outlines several theories - none of which have been proven - about why we yawn:
*To prevent airways in the lungs from collapsing by stretching the lungs and nearby tissues. "This could explain why yawning seems to occur around the time of shallow breathing (when tired, bored or just arising from bed)," Shmerling writes.
*To distribute a chemical that coats the air pockets in the lungs and keeps them open.
*To prepare for an increased level of alertness, especially just after a period of relaxation ("because yawning is associated with stretching of the muscles and joints and an increased heart rate").
*To signal nonverbally that it is time to relax. "Extensive yawning among members of a baboon group signals the time to sleep, typically with the leader ('alpha male') ending the ritual with a giant yawn. For humans, yawning could be a remnant of evolution that communicates the desire to be left alone (or) the need for rest."
*To serve as a warning system that sleep may soon take over.
Most people seem to enjoy yawning: They usually rate it highly on a scale of 1 to 10 in terms of the pleasure it brings.
But those who try to suppress a yawn - the effect of which is called a nasal yawn - rate it as less satisfying, Broughton said.
Excessive yawning can be a sign of disease, such as multiple sclerosis; amyotrophic lateral sclerosis, or ALS, known as Lou Gehrig's disease; and Parkinson's disease.