"Sometimes even mentioning itching will make someone scratch.
"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."
For the study, the researchers put a mouse in an enclosure with a computer screen.
They then played a video that showed 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."
While the mice were watching their friends have a scratch, the researchers monitored their brain activity.
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They found that a brain region called the suprachiasmatic nucleus (SCN) was highly active while the mice were watching the video.
The SCN is a tiny region in the hypothalamus that controls when animals fall asleep or wake up.
When the mouse saw other mice scratching, the brain's SCN released a chemical substance called GRP (gastrin-releasing peptide).
A previous study by the researchers found that GRP is a key chemical transmitter of 'itch signals' between the skin and the spinal cord.
"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."
Dr Chen's team then used genetic engineering to block GRP from travelling from the brain to the spinal cord in some mice.
They found that mice whose GRP chemicals were blocked did not scratch when they saw others scratch.
But they started to scratch normally when they were exposed to itch-inducing substances.
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Dr Chen believes the contagious itch behaviour is something that animals - and humans - can't control.
"It's an innate behaviour and an instinct," he said. "We've been able to show that a single chemical and a single receptor are all that's necessary to mediate this particular behaviour.
"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 hard wired into your brain.
The study was published in the journal Science.
THE EXPERIMENT
For the study, the researchers put a mouse in an enclosure with a computer screen.
They then played a video that showed another mouse scratching.
While the mice were watching their friends have a scratch, the researchers monitored their brain activity.
They found that a brain region called the suprachiasmatic nucleus (SCN) was highly active while the mice were watching the video.
The SCN is a tiny region in the hypothalamus that controls when animals fall asleep or wake up.
SUPRACHIASMATIC NUCLEUS
The SCN is a tiny region in the hypothalamus.
It is responsible for controlling the body's circadian rhythms, the biological body clock used by animals and plants.
The SCN plays a role in controlling when animals wake up and fall asleep, and helps animals regulate their internal body temperature.
