Why do we get goosebumps — when they do nothing at all?

Hidden in your skin is a very small muscle called the arrector pili. When you get cold, the sympathetic nervous system fires a burst of noradrenaline at it. The muscle contracts, the hair is pulled upright, and the skin around each follicle rises into a bump. That is a goosebump.

In our furrier ancestors this was serious equipment. Standing hair traps a layer of air and works as insulation. Facing an enemy, it also makes an animal look one size bigger, which is a cheap way to bluff. A cat's puffed tail and a porcupine's raised quills use exactly the same mechanism.

The problem is that human body hair is far too thin now. Standing it up warms nothing and frightens no one. It has become a feature that was removed from the plan but never deleted from the code — an obsolete command still sitting in the body.

And here is the elegant part: the same wiring does a second job. The shiver you get when a piece of music or a scene in a film moves you — frisson — runs down the same pathway. About half of all people feel it often, and it correlates with the personality trait of openness. So next time you get goosebumps, you can read them as your ancestors applauding.

In one line: goosebumps are old code that no longer throws an error and no longer does anything — except occasionally get triggered by a song.