You get goosebumps when you are cold or when a song moves you. Goosebumps happen when tiny muscles at the base of each hair get shorter.

Then the hair stands up. For animals with a lot of fur, this keeps them warm. Humans have little hair, so people thought goosebumps were just left over from the past. But a 2020 study in mice found that these muscles and nerves also help grow new hair.

Goosebumps appear when tiny muscles called arrector pili muscles, attached to the base of each hair, contract. This happens when the sympathetic nerves, which prepare the body for emergencies, become active because of cold or fear. In furry animals, raised hair traps a layer of air that keeps them warm, and it can also make them look bigger to enemies. Because humans have little body hair, goosebumps have long been seen as a leftover from evolution.

However, a 2020 study at Harvard University found that in mice, these nerves and muscles also send signals to stem cells that make hair. When the mice were cold, these cells started making new hair.

Goosebumps occur when the arrector pili, tiny muscles attached to each hair follicle, contract under the control of the sympathetic nervous system, which is activated by cold or fear. In furry mammals, the resulting raised hair traps an insulating layer of air and can make an animal look larger to a rival. Because humans have so little body hair, the reflex has traditionally been regarded as an evolutionary leftover.

A 2020 study published in Cell, however, suggests it may still serve a purpose. Working with mice, Shwartz, Hsu and colleagues found that the muscle acts as a bridge linking sympathetic nerves to hair follicle stem cells. When cold activated the nerves, the signal also reached these stem cells and triggered new hair growth.

The same system therefore seems to work on two time scales: raising hair immediately and preparing for new fur over the longer term. These experiments relied on genetically modified mice, and it is not known whether the same link works in human skin, let alone whether it could help with hair loss. Goosebumps triggered by music are a different story, since they appear to involve the brain's reward system as well.