You glance across the aisle at the store and see some man vigorously scratching his arm. Suddenly your arm starts to itch and you start scratching too. Is the act "socially contagious" like a yawn, where you can't help doing what others do around you?

Yes, say researchers from the Washington University Center for the Study of Itch (yes, that's a thing), but it goes one step further. They say that contagious itching is hardwired into the brain.

The researchers studied mice to see what went on in their brains when they saw other mice scratching.

“Itching is highly contagious,” said principal investigator and center director Zhou-Feng Chen, PhD, in a statement. “Sometimes even mentioning itching will make someone scratch. Many people thought it was all in the mind, but our experiments show it is a hardwired behavior and is not a form of empathy.”

When a mouse gets itchy

For the study, which was published in the journal Science, Chen and his team placed a mouse in an enclosure with a computer screen. Then they played a video of a mouse scratching.

illustration of mouse scratching when it sees video of mouse scratching When researchers showed a mouse a video of a mouse scratching, the live mouse started scratching too. (Photo: WU Center for the Study of Itch)

Within a few moments, the live mouse would start scratching after watching the video mouse deal with his itch.

“This was very surprising because mice are known for their poor vision," Chen said. "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.”

The brain's role

The team noted that a brain region called the suprachiasmatic nucleus (SCN) in the live mouse was very active when it watched the video of the scratching mouse. When the mouse watched other mice scratching, whether on video or in person, the SCN released a chemical called gastrin-releasing peptide (GRP). In an earlier study, Chen and his team identified GRP as key in transmitting signals of itchiness between the skin and the spinal cord.

“The mouse doesn’t see another mouse scratching and then think it might need to scratch, too,” Chen said. “Instead, its brain begins sending out itch signals using GRP as a messenger.”

When the researchers blocked GRP or GRP receptors in the brains, the mice didn't scratch when they saw other mice scratching. But they were still able to scratch when they felt something that made them itchy.

Chen says he believes that the mice can't control the contagious itching, and that likely points to why we can't either. It's hardwired into the brain.

“It’s an innate behavior 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 behavior."

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.