Tom Stafford, The Social Yawn posted 15 September 2004
The Social Yawn
All animals yawn (see animalyawns.com) and in humans yawning seems to be contagious. Seeing another person yawn, or even just reading about yawning can make you yawn. James Anderson from the University of Stirling gave a lecture in Sheffield last week about yawning - in the introduction he told us that when he lectures on yawning lots of people in the audience, well, yawn. But his talk was only yawn-inducing in the social-contagion sense.
Yawning, it seems to me, may provide us with paradigm case of an automatic behaviour that, moving along the phylogenetic scale, has become co-opted into a quasi-voluntary social signal.
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What am I trying to say and why does it matter? Well, speech - that most human of abilities - is another kind of action that may have begun as an automatic behaviour (mere vocal noises) before being subsequently transformed into a social signal (alarm calls), and then again changed into being a mostly voluntary behaviour.
By looking at yawning we may get clues about how automatic animal behaviours change over evolutionary time into voluntary ones. By looking at yawning contagion we might get clues about how our social nature affects our individual behaviours - and our individual control over those behaviours. In short, if this is the arena where human volition was born and is still mediated we need every 'in' we can get, and yawning may be one.
You can pretty precisely define what a yawn is. The characteristics are instantly recognisable: the screwed-up eyes, the head thrown back and of course - the gaping maw.
In humans a yawn typically lasts around six to eight seconds. At least twenty candidate functions have been suggested for yawning - and we still don't know which ones could be true. It certainly isn't just to do with the levels of oxygen in our lungs - some very thorough scientists have done experiments involving raising the ambient levels of carbon dioxide and found that it didn't increase people's frequency of yawning. We do know that it is found in all vertebrates and that it develops early - even 10 week old human foetuses yawn.
Like a lot of behaviours held in common by many species (it has even been suggested that fish yawn!), and which develop early in the lifespan, yawning seems to be controlled in the central, ancient, part of the brain - the brainstem. (And let’s note here, that in monkeys vocalisations also seem to be brainstem controlled).
Studies have shown that in old-world monkeys yawning is more common among males, and among those monkeys higher up the dominance hierarchy. It could well be that in these creatures, yawning has become a social signal of a different kind than in humans - a display of the canines and hence a warning to anyone thinking of causing trouble. The reports by some researchers that baboons are likely to yawn before fights, and possibly also likely to turn their heads in profile to the animal they are yawning at - better showing off those vicious teeth - would support this idea.
Another possible function for yawning among primates - including ourselves - is that it fulfils a social coordination role. A way for a group to signal to itself something like "time for bed" or "we're bored, let's do something else now". It's not clear, however, why yawning would take on this role, nor, indeed, is it certain that a group of monkeys should all sleep at the same time.
Even in monkeys, yawning was making the transition into being a semi-voluntary behaviour. Dr Anderson reported evidence that yawning can be trained into macque monkeys (using rewards for yawning behaviour). So, for primates at least, yawning is not only a reflex, but perhaps an example of a multi-purpose behaviour or a social signal. Although I wonder whether control-over-yawning was itself adaptive, or whether some more general increase in voluntary (cortical?) control allowed control-over-yawning as a by product.
Chimpanzees, closer relatives of ours than monkeys, when shown videos by Dr Anderson of other chimps yawning, themselves yawned - showing that the yawning contagion effect found in humans evolved before whatever happened that made us human.
In humans, Dr Anderson had done experiments showing that, unlike adult chimpanzees, pre-school children don't catch yawning from others. This would fit with research in child psychology which suggests that, until around the age of three or four, children aren't able to think about other people's states of mind. They don't have what many people would call empathy (and what psychologists call 'Theory of Mind'). One aspect of this lack may be that they don't socially mimic others like adults do, and hence don't catch yawns.