A study reported in the journal Child Development measures the incidence of contagion in yawning among a group of young children, some of whom were autistic. Conducted by Molly Helt, a graduate student in clinical psychology at the University of Connecticut, Storrs and colleagues, the study shows that yawning becomes contagious for children at the age of four, but is less contagious for autistic children than for non-autistic children.

"Emotional contagion seems to be a primal instinct that binds us together," said Helt. "Yawning may be part of that." She conducted the study after trying and failing to get her own autistic child to clear his ears on a plane by repeatedly yawning at him.

"The fact that autistic kids don't do it might mean they're really missing out on that unconscious emotional linkage to those around them," says Helt. "The big thing people try to figure out in infant development is how we become humans who understand that humans have minds that are different from ours," she added. "Autistic people never sort of seem to understand that."

Spontaneous yawning begins in the womb at the age of 11 weeks, and continues throughout life. All vertebrates yawn, although yawning contagion has been demonstrated only in humans, dogs and the great apes.

There are multiple theories about the reason for yawning, and why it should be contagious, although it seems obvious that it is a form of communication as well as having some bodily utility in raising altertness and general readiness. Otherwise, why yawn when you are on your own? Specifically, Helt says, it could diffuse stress after a period of being on high alert and spread a feeling of calm through a group. In baboons, at least, yawning involves an aggressive display of teeth. It is easy to believe that among a somnolent group of hunter-gatherers, there would be utility in behaviour that led to a greater degree of alertness.

Whatever the reason for contagious yawning, at least it is sure that it is a display of empathy, and there are other contagious behaviours in a group context which employ the principle of empathy, including laughing, crying and angry aggression. Such behaviours are entirely involuntary; with the development of the self and 'conscious' self-awareness in humans come facial behaviours which are under a mixture of conscious (voluntary) and unconscious (involuntary) control, and these can also be contagious. Smiling, sadness, surprise and sympathy are examples of states which we may express entirely 'naturally' or which we may at least partially construct to suit our conscious inter-personal agenda at a given moment. In many social (group) situations such states are contagious within the group, indeed we may deliberately set out to use them to influence the behaviour of the group.

It's interesting to think about plays and actors in this context. A good actor (something rare in our society) is able to construct emotional displays very effectively. How do they do it? Is it by suppressing their self-awareness, so that the voluntary mode of expression has free rein? Or is it because they are so practised in controlling their facial musclature (good at mimicry, if you want) that they can convincingly imitate genuine displays?

Read more in Chapter Three of Agent Human by Michael Bell, The Evolution of Social Consciousness in Humans.