PHILADELPHIA — Think about someone yawning. Jaws gape. Lips spread wide. Arms stretch. There's that quick inhale and the long aaaahhhhh. Yawning yet?

Don't worry. We're not offended. In fact, according to a new Drexel University study, you're just being kind.

Yawning is an ancient, primitive act. Humans do it even before they're born, opening wide in the womb. Some snakes unhinge their jaws to do it. One species of penguins yawns as part of mating.

Only now are researchers beginning to understand why we yawn, when we yawn and why we yawn back.

Steven Platek, a professor of cognitive neuroscience at Drexel, studies the act of contagious yawning, something done only by people and other primates.

In his first study, published in 2004, the former jazz guitar major (he wasn't doing so well and switched to psychology), used a psychological test to rank people on their empathic feelings.

He found that participants who didn't score high on compassion didn't yawn back.

"We literally had people saying, 'Why am I looking at people yawning?'" Platek said. "It just had no effect."

For his second study, Platek put 10 students in an MRI machine as they watched videos of people yawning. That study was published in February with Feroze Mohamed, associate director of Temple University's Functional Brain Imaging Lab, and Gordon Gallup Jr., of the State University of New York at Albany.

When the students watched the videos, what reacted was the part of the brain that scientists believe controls empathy — the posterior cingulate, located in the brain's middle rear.

"I don't know if it's necessarily that nice people yawn more, but I think it's a good indicator of a state of mind," said Platek. "It's also a good indicator if you're empathizing with me and paying attention."

Now he's studying yawning in those with brain disorders, such as autism and schizophrenia, in which victims have difficulty connecting emotionally with others.

There are few researchers who study the basic physical act of yawning. Robert Provine, a psychology professor at the University of Maryland, is one of the few, and is so well-known for his work that he describes himself as "a yawn stimulus." (In a nice way.)

Last month, he published an analysis of yawning myths, research and physiology in Scientific American.

The physical root of yawning remains a mystery. Some researchers believe it's coordinated by the paraventricular nucleus in the hypothalamus of the brain, the tiny part that controls basic functions like breathing and heartbeat. But an MRI can't see something that small, so no one really knows for sure, Platek said.

Part of the problem with yawning research is that it's hard to get government funds to cover the work, Platek said. Both researchers received money from their universities for their studies.

"It's hard to get money for basic science," Platek said. "It's not like this is curing cancer."

The original theory was that we yawned when we needed more oxygen. But that's not true, Provine said, pointing to his study in which volunteers inhaled pure oxygen, but continued to yawn away.

Platek, who began studying yawning to appease a curious thesis adviser, said he loves to watch people try to stop yawning while talking to him about that very subject.

"They do the inhale. They do the hard bite, kind of clenching their teeth," he said. "They're trying not to be rude and I'm thinking 'Sweet! They're listening to me.'"