Cover your mouth if you want to, but it won't keep the next guy from "catching" your yawn.
Contagious yawning is real, scientists say, and it's "probably programmed into us," according to Dr. William Broughton, director of the Sleep Disorders Center at the University of South Alabama Knollwood Hospital.
The action of a mouth opening is not what compels others to yawn, Broughton said. Studies have demonstrated that showing someone a photo of a wide-open mouth does not induce a yawn.
Conversely, holding a hand over the mouth while yawning doesn't prevent it from being contagious, Broughton said.
Contagious yawns appear "basically to be a visual response," Broughton said.
Between 40 and 60 percent of people who watch videos or hear talk about yawning also end up doing the deed, according to Nature News Service.
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.