According to research performed at New York State University, between 40 and 60 percent of the population seems to find yawning contagious.

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.

Also, Ronald Baenninger, a professor in the psychology department at Temple University in Pennsylvania, has conducted a study to see if the contagion works between species. He and his 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.

Dr. William Broughton, director of the Sleep Disorders Center at the University of South Alabama Knollwood Hospital, said 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. Also, holding a hand over the mouth while yawning doesn't prevent it from being contagious, Broughton said.

A scientific look at the issue is this: The Finnish government recently funded a brain scanning study that shows yawn contagion is largely unconscious. Wherever it might affect the brain, it does not affect the known brain circuitry for consciously analyzing and mimicking other people's actions; this circuitry is called the "mirror-neuron system."

The researchers found that yawning seems to activate at least one brain area, which is called the superior temporal sulcus. Even more relevant was the apparent deactivation of another brain area, the left periamygdalar region. The more strongly a participant reported wanting to yawn in response to another person's yawn, the stronger was this deactivation.

Studies on physical brain activity related to yawn contagious seems to be inconclusive.

Some scientists believe yawning came to be a method to communicate levels of alertness to each other and coordinate sleeping times.

It's also been suggested contagious yawning could be a result of an unconscious herding behaviour, a way to communicate to those around us, similar to when flocks of birds take flight at the same time.