More recent research has revealed that chimpanzees and even dogs can catch yawns from those around them, including from humans, but little is known about why we yawn and why it appears to be so infectious.
Psychologists at the University of Stirling, however, have now discovered that infants and young children are not prone to the contagious aspect of yawning. Instead, they only ever yawn spontaneously.
Dr Jim Anderson, a reader in psychology at the University of Stirling who led the research, which is published in the Royal Society journal Biological Letters, believes the findings will help to shed new light on how the human brain develops as we grow up and what makes us yawn.
I like the brain-cooling theory, which I found summarized at http://www.sciencedaily.com/releases/2007/06/070621161826.htm. The researchers, Gallup and Gallup, think the brain works better when cool, yawning is an attempt to improve alertness, and just maybe it is contagious to promote group vigilance when there is danger. I really like the idea of air cooling the brain.