Scratching can be catching, new research has shown.
Just like yawns, scratching an itch is contagious, say scientists.
The sight of someone trying to get rid of an itch can lead us to scratch ourselves - without even knowing it.
Researchers tested 30 volunteers to see if visual cues could trigger itching and compel them to scratch.
Participants were shown "itch-related" images such as ants, fleas, or skin rashes, or neutral photos including butterflies and healthy skin.
Some pictures only showed a possible cause of itching, such as ants crawling on the skin, while others depicted itchy people scratching or washing their hands.
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As volunteers looked at each image, they were asked how itchy they felt. The scientists also recorded the number of times participants scratched themselves.
The results, published in the British Journal of Dermatology, showed that visual images alone can trigger itch sensations.
But it was watching another person scratching, rather than just seeing the cause of the itch, that led the volunteers to scratch themselves.
Lead scientist Professor Francis McGlone, from Liverpool John Moores University, said: "The results of the present study confirm that visual cues pertaining to itch-related events are effective in transmitting the sensation of itch from the visual to the somatosensory domain (the body's system relating to the sensation of touch) and provoking a scratch response.
"The results suggest that, whereas the sensation of itch may be effectively transmitted by viewing others experiencing itch-related stimuli on the body, the desire to scratch is more effectively provoked by viewing others' scratching.
"Our findings may help to improve the efficiency of treatment programmes for people suffering from chronic itch. Knowing the specific triggers of an individual's chronic itch and how visual stimuli translate to the physical may also provide insight into the mechanisms of 'psychosomatic itch', in which there are no physical triggers."
Nina Goad, of the British Association of Dermatologists, said: "Itch is often the worst symptom for people with skin disorders, and any research into its causes that may lead to new methods of alleviation will be greatly welcomed by the millions of skin patients.
"Combining elements of psychology with dermatology is an increasingly important area of research."
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