Why do we get goosebumps in the cold when our body hair is so short?

This muscle is called the arrector pili. One end is attached to the hair root and the other sits at an angle just beneath the skin, so when it contracts, a flattened hair stands up and that spot bulges. It isn't a muscle you control at will, so it moves on its own when your body senses cold and relaxes within a few minutes once you get somewhere warm. Your palms and soles have no hair, so they don't get goosebumps.

Animals with thick fur raise it in the cold to build a deeper layer of air between the hairs. That air keeps body heat from escaping. Human body hair is short and sparse, so raising it does almost nothing to keep us warm.

So the idea that goosebumps warm you up doesn't hold for people. What actually makes heat in the cold is shivering. We have less hair, but the muscles and the signals remain, so the response still happens as it always did, even if it's of little use.

The nerves that signal this muscle are the same ones that speed up your heart when you're tense. That's why, even without any cold, you get goosebumps when you're on edge, like when listening to a scary story. Getting chills at the peak of a favorite song or during a moving scene comes from the same signal.

Most people can't produce goosebumps at will, but a 2018 study found and described 32 people who said they could raise them on their own. It's very rare.

In 2020, a Harvard University team showed in mice that this muscle and its nerve sit right next to the stem cells that regrow hair. Mice that spent two weeks in the cold regrew their hair sooner. Since these were mouse experiments, it's too early to say the same holds for people.

If goosebumps bother you, the quickest fix is to roll down your sleeves and get somewhere warm. Bumps that stay itchy or red for a long time even somewhere warm may be a skin problem other than goosebumps.