Why do we get goosebumps?

The answer

Goosebumps (piloerection) are caused by the contraction of tiny arrector pili muscles at the base of each hair follicle, triggered by the sympathetic nervous system in response to cold, fear, or strong emotions. In our fur-covered ancestors, this reflex served two purposes: raising the fur trapped a layer of insulating air to retain heat, and in threatening situations, puffed-up fur made the animal appear larger and more intimidating to predators. Although the reflex persists in humans, our sparse body hair makes it largely functionless. Interestingly, recent research has revealed that goosebumps may not be entirely vestigial: the sympathetic nerves that trigger piloerection also stimulate stem cells in the hair follicle, potentially promoting hair growth, suggesting the reflex may retain a biological role beyond its original thermoregulatory and defensive functions.