Yawning has to be tops on the list of behaviors that are easily spreadable among people. We’ve all experienced the contagious yawn, so there's no need for smarty-pants scientists and their fancy studies to tell us that one.
But itching is a subtler behavior, with a less easily observable "communicability"—though you’ve probably felt an irresistible urge to scratch yourself if you catch someone chatting about the lice infestation they had in second grade, or recounting a particularly brutal case of poison ivy.
So researchers from the Washington University School of Medicine in St. Louis decided to put the "contagious itch" theory to the test on a bunch of mice. They discovered that the urge to replicate someone else's scratching, is actually hardwired into the brain, and it's not a choice or even a conscious response.
In the new study, researchers showed mice a video of other rodents scratching, and noticed that the live mice would start scratching right away. The researchers they found that a brain region called the suprachiasmatic nucleus lit up when the itchy mouse vid was shown, and that activation released a messenger chemical in the brain that normally transmits itchy signals between the skin and the spinal cord. When watching the video or other mice scratching, the brain started directly releasing the chemical, causing the urge to itch.
From there, the scientists blocked the chemical or its receptor sites. When that happened, the mice didn’t show any of the socially contagious itching behavior, but would still scratch if administered typical physically irritating stimulus—suggesting that the messenger chemical really is the cause of that psychosomatic scratching.
“Itching is highly contagious,” said lead author Zhou-Feng Chen, Ph.D., director of the Washington University Center for the Study of Itch (for real). “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. 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.”