How contagious is yawning? Enough that I'm yawning right now just from writing about it. Admittedly, I'd never given it much thought, but I always kind of assumed that contagious yawning—that tendency to pick up a yawn from the people around you and pass it on like the evil videotape in The Ring—was something all humans were susceptible to.
But that seems to be over-generalizing a bit. Everybody spontaneously yawns. Even fetuses. And around 40-60% of us will yawn if someone else around us yawns. But that's neurotypical adults.
There's a growing collection of research showing that very young children, and people with autism spectrum disorders, don't succumb to contagious yawning. In fact, contagious yawning is currently thought to be related to the development of empathy and the ability to distinguish our own mental states from those of others. One study, done in 2007, even found that psychology students—who presumably spend a lot more time than average thinking about other people's mental states—were more susceptible to contagious yawning than engineering students.
Oh, and yawning, in general? Yeah, nobody really knows why we do that.