Why do we get goosebumps?

At the base of every hair sits a tiny muscle called the arrector pili. When it contracts the hair stands upright and the skin around its base bunches into a small bump — those bumps are the goosebumps.

That muscle is driven by the sympathetic nervous system, which responds both to cold and to strong emotion. Fear, excitement and a moving passage of music therefore trigger the same reaction — different stimulus, one nerve pathway.

In humans it achieves nothing useful, because our hair is too fine. In furrier animals it does: raised fur traps a layer of insulating air, and in a confrontation it makes the animal look bigger. A cat’s tail puffing up is the same mechanism.

Certain drugs and illnesses can raise goosebumps too, because anything that stimulates the sympathetic nervous system reaches the same muscle. Normally the response fades within seconds, since those muscles do not hold a contraction for long.