Much time has been spent, over the past century, on working out exactly what it is about the sound of fingernails on a blackboard that's so unpleasant. A new study pins the blame on psychology and the design of our ear canals. They found that disturbing sounds do cause a measurable physical reaction, with skin conductivity changing significantly.
A powerful psychological component was identified. If the listeners knew that the sound was fingernails on the chalkboard, they rated it as more unpleasant than if they were told it was from a musical composition. Even when they thought it was from music, however, their skin conductivity still changed consistently, suggesting that the physical part of the response remained.