Hanging out with Steve Platek will make you yawn. He'll get you thinking about yawning, reading about yawning, and sooner or later, your mouth's gaping. You can't help it. "My favorite way to induce a yawn," Platek says, "is a video clip of a good yawner paired with yawn audio." Platek, a cognitive neuroscientist at Drexel University, alternately describes yawning as "a primitive unconscious mechanism" or something that's "sweet," "totally cool" or "awesome." And he's finally figuring out why it's contagious.

Scientists (and everybody else) have known for decades that yawns are contagious, but they've never known why. Some think it's an unconscious mirror effect -- someone yawns, you yawn in response almost like a reflex. But Platek says he thinks it has to do with empathy. The way he sees it, the more empathetic you are, the more likely it is that you'll identify with a yawner and experience a yawn yourself. In a recent study, Platek looked at contagious yawning in people with "high empathy," "low empathy" and everything in between. He found that higher empathy meant more yawn-susceptible and lower empathy meant more yawn-immune.

But that wasn't proof enough. So Platek put volunteers in M.R.I. machines and made them yawn again and again to pinpoint the areas of the brain involved. When their brains lighted up in the exact regions of the brain involved in empathy, Platek remembers thinking, "Wow, this is so cool!"

Some yawning researchers -- of which there are few -- have identified many types of yawns. There's the contagious yawn, the I'm-tired yawn and the I-just-woke-up yawn. There's the threat yawn, which is the my-teeth-are-bigger-than-yours yawn that's so popular with primates. ("People do it, too," says Platek, "but unfortunately, we don't have scary teeth anymore.") There's also the sexual yawn. (One scientist claims that yawns are used in seduction.)