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Science uncovers the mystery behind itching and scratching
Researchers are beginning to understand why an itch is so hard to ignore and why scratching brings relief, though the behavior may have evolved to signal danger to others.
The biology of itch
For years, the science of itch remained poorly understood. Recent discoveries, however, reveal that specific neurons and neural pathways transmit the sensation, paving the way for new treatments. Scientists believe mammals developed itching as a reflex to expel harmful pathogens or avoid toxic substances in their environment.
Yet, the psychological dimension of itch remains puzzling. One example is contagious scratching-where merely seeing someone else scratch triggers an itchy sensation in observers.
Contagious itch: A shared experience
In a 2011 study, Gil Yosipovitch, a dermatologist at the University of Miami's Miller School of Medicine, tested this phenomenon. Healthy participants and individuals with atopic dermatitis (AD), a chronic itch condition, watched videos of people scratching or sitting still. Some received histamine injections to induce itching, while others got a harmless saline solution.
Both groups reported increased itching after viewing scratching videos, but the effect was stronger in AD patients-82% felt itchier. The study also found that rhesus macaque monkeys scratched more after watching videos of other monkeys doing the same, suggesting contagious itch isn't unique to humans.
Evolutionary instinct: Avoiding the itchy
Research suggests mammals may have evolved to avoid itchy individuals. Brian Kim, a neuroimmunologist at New York's Icahn School of Medicine, observed that mice moved away from scratching cage-mates but approached those in pain to groom them. This behavior likely stems from an evolutionary adaptation to avoid parasites or infections.
"I think instinctively it's survival. If I see someone scratching on the subway, I stay away. It's hardwired into our nervous system that scratching may signal contagion," Kim said.
Brian Kim, Icahn School of Medicine
This avoidance may explain why chronic itch sufferers often experience shame, anxiety, and depression. Stress can worsen itching, creating a vicious cycle.
The pleasure and pain of scratching
Why does scratching feel so satisfying? One theory suggests it triggers mild pain signals that temporarily override the itch. Scratching also releases serotonin, a neurotransmitter linked to pleasure. However, the exact mechanisms remain unclear.
"The skin transmits information to the brain in a way that can't handle multiple sensations at once. In chronic itch sufferers, scratching becomes addictive because it triggers pleasure neurons," said Marlys Fassett, a dermatologist at the University of California, San Francisco.
Marlys Fassett, University of California, San Francisco
Despite the relief, experts warn against advising chronic itch patients to stop scratching. "It's like telling someone to stop yawning-it's a reflex," Yosipovitch noted.
Public health implications
Scratching may offer some protection against infections. For example, scabies outbreaks are common in nursing homes where immobile patients can't scratch. This reinforces the idea that frequent scratching could signal a transmissible condition, prompting avoidance.
