It starts with a tingling, itch-like sensation at the back of the mouth, develops into a deep intake of breath, followed by an equally resonant exhalation – often accompanied by a satisfying groan. To the medical profession it is a “brainstem-mediated bodily response”; to anyone else it’s a yawn.
Most animals with any backbone yawn spontaneously but only humans, chimps and possibly some species of monkey suffer from contagious yawning – when the sight or thought of someone else yawning starts a chain reaction.
Why yawning should be infectious has foxed some the greatest minds in science but the latest study into the topic suggests it may have something to do with emotional empathy – we yawn when we see someone else yawning because of our need to em-pathise with other people.
Atsushi Senju of Birkbeck College, part of London University, and colleagues from Japan investigated infectious yawning in autistic and non-autistic children. They found that autistic children did not experience contagious yawning. Autism is known to be a developmental disorder of the brain which results in children being unable to form normal emotional ties with the people they meet. Some experts suggest the condition is the result of an inability to empathise with other people’s emotional states.
The finding that autistic children do not respond to contagious yawning – although they yawn spontaneously just like any other children – suggests that emotional empathy may be the underlying reason why yawns are infectious. As the scientists say in their study published in the journal Biology Letters, contagious yawning is impaired in autistic children and may relate to the fact that these children also find it difficult to empathise. “It supports the claim that contagious yawning is based on the capacity for empathy,” Dr Senju said.
“This study is the first to demonstrate an impairment in contagious yawning in children with autistic spectrum disorder. To the best of my knowledge this is the first time that anyone has demonstrated that a neurodevelopmental disorder can lead to an impairment specific to contagious yawning,” he said.
Almost every vertebrate animal studied, from fish to cats and dogs, yawns spontaneously as part of an evolutionary, ancient, reflex response. Only humans and chimps, and possibly the macaque monkey, have been shown in scientific tests to yawn infectiously. Some scientists have suggested that yawning brings about an increase in the oxygenation of the brain’s “yawn centre” and is designed to maintain levels of alertness, particularly in times of stress. This may explain why, for instance, yawning is often seen when people are waiting for something stressful to happen. It is common among athletes just before their event begins, or indeed among students just prior to an exam.
But this does not explain why yawning should have evolved into an infectious activity. One theory is that it has something to do with the fact that humans once lived in troupes, just like chimpanzees, and that it was important to coordinate sleeping, and so serving as a social signal.
The latest study, however, pinpoints the fact that contagious yawning appears to involve the very human trait of emotional empathy. Humans, uniquely, are able to imagine what someone else is thinking or feeling. This is at the heart of empathy, and it is a trait that autistic children unfortunately lack – and so may explain why they are immune to the infectiousness of yawning.