Study reveals why seeing others scratch makes you itch, too

ST. LOUIS — Ever wonder why when someone near you starts scratching themselves, you find yourself suddenly itching like crazy too? A new study finds there’s science behind the itch phenomenon.

Researchers at the Washington University School of Medicine in St. Louis discovered that socially contagious itching is something that’s actually “hardwired” into our brains.

To test out this concept, the team put a mouse in an enclosure containing a screen, and then played a video showing another mouse scratching itself. The study’s principal investigator, Zhou-Feng Chen, PhD, director of the Washington University Center for the Study of Itch explains what happened next in a university news release.

Cat scratching an itch
Why do you always start to itch when you see someone else scratching? A new study reveals the science behind the phenomenon.

“Within a few seconds, the mouse in the enclosure would start scratching, too. This was very surprising because mice are known for their poor vision,” he says. “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.”

After that, the researchers took the study a step further and determined the chemical receptor in the brain responsible for the contagious reaction. They found that the suprachiasmatic nucleus (SCN) — the part of the brain that controls sleep in animals — demonstrated significant activity after the rodent saw the mouse itching. They determined the SCN released a substance called gastrin-releasing peptide (GRP). In a previous study, Chen’s team found the substance acts as a messenger that sends signals from skin to spinal cord that creates the body’s itching sensation.

Essentially, when the mouse saw the video, its brain released GRP and made it feel itchy. They confirmed this by blocking GRP or its receptor and discovered that once blocked, the mouse didn’t scratch when the video was played again. Substances that would cause the mouse to itch were tested on it as a control and sure enough, the rodent scratched when in contact with those substances.

Ultimately, Chen says the itching feeling we get from seeing others scratch is something we can’t control.

“Itching is highly contagious. Sometimes even mentioning itching will make someone scratch,” he says. “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.”

The study was published last month in the journal Science.