Why

In the cinema, it is hot and the film is boring. Someone a few seats away is yawning. You cannot resist doing the same.

At dinner with friends, you cannot hide your tiredness. Each time you yawn, you cover your mouth and mumble, ‘Sorry.’

Why do we yawn at these moments?

Yawning is not just deep breathing to give us some fresh air. It has a particular pattern of three phases: a long drawingin of breath, a peak and then a quick exhalation. At the same time, we may or may not stretch our trunk and limbs. Either way, afterwards we feel relaxed and content.

Greek physician Hippocrates thought yawning helped clear fever out of our systems in a way similar to how a chimney gets rid of smoke. Only in the 1980s did medicine, especially neuroscience, begin to take a stronger interest.

But the basic question of why we yawn at all is still not fully understood. It seems to improve our vigilance and may have some distant link with helping survival. At least we now know that it has no role in getting rid of fever.

Everyone has seen newborn babies yawning. But does it start even earlier? Molly Helt at the University of Connecticut gives a confident ‘Yes’ to this question.

‘Foetuses yawn quite spontaneously in the womb,’ she says. ‘Researchers in the Netherlands used ultrasound to observe such yawning. They found it happens as early as 11 weeks after conception. Foetuses also yawn and stretch their bodies at the same time, just like adults.’

When we see someone yawn, we find it hard not to succumb ourselves. ‘Oh look, you’ve started me off now’ is a common response. This urge to mimic yawning happens not only with us but with great apes, too.

Do young children ‘catch’ the yawning of others, as adults seem to? Helt says, ‘We looked at yawning in infants and children. Previous studies had examined how copied yawning is triggered by videos. This detected such mimicking at about the age of five. When we used the stimulus of real people yawning, rather than video clips, we found that infants as young as two copied the behaviour.’

In fact, researchers used this contagious effect to try to identify key features of yawning in adults. They found that some essentials seem to bring about copying more than others.

Robert Provine of the University of Maryland says, ‘We normally think of the most essential feature of a yawn as being a wide-open mouth.’ But this alone was not enough to trigger yawning. It looks too much like someone shouting or singing at full pitch.

In one experiment, researchers showed volunteers representations of a yawning mouth but without the context of the whole face. Participants were as unlikely to yawn afterwards as when shown an isolated mouth smiling. But if volunteers were shown a whole animated face yawning, but with the mouth masked, they were just as likely to yawn in response as when they could see the mouth.

Provine explains, ‘It seems that when yawning is triggered, we take in the context of the whole face. This includes the gaping mouth but also other features like squinting eyes. Other clues in real life can come from stretching our arms or tilting our head back. Yawning includes a whole package of facial expression and sometimes our body posture and movement as well.’

Why is this strange behaviour so contagious? It appears that certain nerve networks are brought into play when we copy yawning, and these relate to the circuits activated when we feel in tune with the emotions of other people.

It is as if yawning is some deep-seated copying behaviour, showing empathy.

Dr Michael Farrell is a psychologist. His book Controversies in Schizophrenia is out in August (Routledge)