Fear not only causes goose bumps, but it also makes our hair stand on end, and both reactions are related.
When we are frightened, our bodies draw blood away from our extremities, like skin, and redirect it to support our vital organs.
As a defense against this tendency, our very hairy primitive ancestors developed an evolutionary response to keep the body warm.
When blood is drawn away from the skin, it triggers tiny muscles that tighten the skin and force body hair to stand up to trap heat.
This reaction causes stiffening where we once had a lot more body hair, and because the raised flesh looks like the skin of a plucked goose, we call the result goose bumps.