Like yawning and coughing, scratching can be infectious. You see someone else scratching and soon you feel itchy yourself. Itch transmission has been investigated (16)---- showing people pictures of fleas and ants. This can make them scratch (17)---- watching someone else scratching produces the strongest response. A possible clue as to why this happens may be (18)---- when a drop of histamine, a substance given out in the body in response to an allergy, is dropped on someones skin. This makes them itchy, but they scratch all over, not just where the drop was placed. This suggests a mechanism that makes us hypersensitive to skin sensations and lowers the threshold for wanting to scratch, which (19)---- the cause of infectious scratching. Why do we have it? Other primates also behave this way (20)---- several reasons. One theory is that it evolved in social species to help them notice and deal with parasitic infections.
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