Check out the woman on the left and try not to yawn. Go on, give her a good ten seconds of your time. I mean, it’s Friday and it’s been a long week—I can’t blame you. Really—try your absolute hardest not think about yawning as you read this post! C'mon, you know you can do it—you've been dared before, and you always fail miserably. NO YAWNING!

Chances are you've already let out an extended, eye-moistening, feel-good yawn or two at this point. I've personally counted six of my own since starting this post.

We've all heard that "yawning is contagious"—but why? In this busy world, we don't sleep as much as we should. Gallup Polls in recent years have found that 56% of Americans report drowsiness as a daytime problem, and 34% of us are "dangerously ." Does seeing someone yawn remind us that we, too, are exhausted are must follow suit?

That may be part of it, but the true reason may go much deeper. As it turns out, yawning may have ancient roots in social .

I'm not surprised by this claim; when I let out a mistakenly too-loud yawn in class, I tend to get sympathetic nods or "I hear ya"s. Similarly, when I see someone else yawn, I feel validated in my sleepiness or boredom.

All vertebrates yawn; in fact, only humans, chimps, and possibly dogs do so contagiously.

The hypothesis that this behavior may be grounded in bonding actually came from an observation by Molly Helt, lead researcher on the study responsible for this finding published in Child Development. Helt, while on a plane with her son, tried to get him to clear his ears by repeatedly yawning. Her son never responded with his own yawns.

To determine when this possible social yawning begins developmentally, Helt recruited 120 healthy kids between the ages of one and six. Grouped by age, she read 10-minute stories to the children, yawning intentionally every 90 seconds. A camera recorded whether the children were watching her and if they, too, yawned.

Helt and colleagues also repeated the experiment with 28 autistic children aged between 6 and 15 years.

One 2-year old and two 3-year olds yawned back; however, a dramatic leap occurred among the 4-year olds, where nine of 20 responded with yawns. This rate was similar for older age groups.

Interestingly, in children with mild autism, contagious yawning occurred only half as often; those with severe autism never returned the behavior.

Helt explains, "The fact that autistic kids don't do it might mean they're really missing out on that emotional linkage to those around them."

Frank Sinatra once said, "Never yawn in front of a lady." But if she's pretty and you want her number, maybe it's the best way to bond with her—that is, if she is flirtatious enough to return one.

Helt MS, Eigsti IM, Snyder PJ, & Fein DA (2010). Contagious yawning in autistic and typical development. , 81 (5), 1620-31 PMID: 20840244