Bored? Sleepy? Lack of oxygen? Who knows?
The Library of Congress posted an interesting analysis of this question in Everyday Mysteries: Why do we yawn? They conclude that it may serve a social function and/or a physiological one, which leaves the door pretty wide open.
The article claims that “generally speaking, we cannot yawn on command.” I find that I can yawn whenever I choose to, which is handy on airplanes. Do others find that they lack conscious control over yawning? (Stifling a yawn, however, is really difficult!)
Apparently 42-55% of non-autistic adults find yawning contagious. I’m surprised that the percentage isn’t higher. Do you find that the picture of the man yawning above makes you want to yawn? Try doing a google image search on “yawn” and see if you can escape the power!
As a bonus, I learned two nifty new words while reading this article: