Scratching, yawning and other ‘socially contagious’ behaviours have long puzzled scientists – if someone else has an itch, why should we?
But now it appears a key part of the mystery has been solved.
In a study on mice, researchers found that the mere act of seeing another individual scratch prompted the brain to release a chemical that helps to communicate an itch signal from the skin to the spinal cord.
So rather than consciously choosing to scratch or feeling the need to do so because of some sense of empathy, the response is essentially hard-wired into the brain.
And, when it is triggered, itches that the brain might otherwise have ignored suddenly become much more noticeable.
In a paper in the leading journal Science, the researchers, of Washington University in St Louis and the Fourth Military Medical University in Shaanxi, China, said it would be “of interest” to discover if the same process was behind yawning and other behaviour that seems to catch on.
Dr Zhou-Feng Chen, director of the Washington University Centre for the Study of Itch, said the study showed a physical reason, rather than a mental one, behind the phenomena.
“Itching is highly contagious. Sometimes even mentioning itching will make someone scratch,” he said.
“Many people thought it was all in the mind, but our experiments show it is a hard-wired behaviour and is not a form of empathy.
“It’s an innate behaviour and an instinct. The next time you scratch or yawn in response to someone else doing it, remember it’s really not a choice nor a psychological response, it’s hardwired into your brain.”
In the study, a mouse was played a video of another mouse scratching.
“Within a few seconds, the mouse in the enclosure would start scratching too,” Dr Chen said.
“This was very surprising because mice are known for their poor vision.
“They use smell and touch to explore areas, so we didn’t know whether a mouse would notice a video.
“Not only did it see the video, it could tell that the mouse in the video was scratching.”
Part of the mouse’s brain called the suprachiasmatic nucleus (SCN) – which controls falling asleep and waking up – was particularly active when it saw another mouse scratching.
And it was found to release a chemical known as GRP when this happened. Previous research by Dr Chen found GRP helps transmit itch signals from the skin.
“The mouse doesn’t see another mouse scratching and then think it might need to scratch, too,” Dr Chen said.
“Instead, its brain begins sending out itch signals using GRP as a messenger.”
To provide more proof of the link, the researchers blocked the release of GRP in mice, then showed them the scratching video – and they did not scratch. However, when they were exposed to itch-inducing substances, they did scratch.
And, when the SCN was artificially stimulated, the mice also started scratching.
“We’ve been able to show that a single chemical and a single receptor are all that’s necessary to mediate this particular behaviour,” Dr Chen said.
In the paper in Science, the researchers added: “It will also be of interest to determine whether SCN sub-circuits may mediate other types of socially contagious behaviours, such as yawning or empathy for pain.”
Various theories have been advanced to explain yawning.
Some have claimed it is designed to give an extra hit of oxygen when we are feeling tired or sluggish, but studies have found they do not actually result in a rise in the amount of air going into the lungs.
It has also been suggested that yawns help cool the brain by increasing the blood flow and taking in a big gulp of air, enabling us to think more clearly. The brain’s temperature, which has an optimum level much like a computer, rises when we are tired.
But neither of these theories explain why yawning seems to encourage other people to do the same.
This has prompted researchers to claim it is a means of social bonding, with evidence suggesting it is more likely to spread among close relatives.
A study published last year suggested women are more likely to yawn when others do because they have a greater sense of empathy.