Why do we get goosebumps?

Goosebumps, known medically as piloerection, occur when the arrector pili muscles at the base of body hairs contract, causing the hairs to stand on end and dimpling the skin. In furry mammals this traps an insulating layer of air when cold and makes the animal look bigger when threatened. In humans it is largely vestigial — our sparse body hair gains little warmth or bulk — yet the reflex persists, controlled by the sympathetic nervous system. That is why we also get goosebumps from powerful music, awe, or fear, when adrenaline surges through the body.