Why do we get goosebumps?

Each hair on your body has a tiny muscle attached, the arrector pili, and when cold or strong emotion triggers your sympathetic nervous system, those muscles contract and stand the hairs up, dimpling the skin. In furry animals this does real work: raised fur traps an insulating air layer for warmth, and puffs the animal up to look bigger and more threatening to a rival or predator, think a startled cat doubling in size.

On humans, who lost most of that fur, the reflex fires the same and accomplishes almost nothing, which makes goosebumps a textbook evolutionary vestige, a working mechanism attached to hardware we no longer have. The emotional trigger is the interesting survivor: a chill down the spine at powerful music or fear runs the identical circuitry as cold, which is why awe and terror literally raise the same bumps. Recent research even found the nerves controlling these muscles help regulate hair-follicle stem cells, so the leftover may not be entirely idle after all.