Why do we get goosebumps when we're scared?

Chris - Goose bumps or goose pimples are the bulges produced by your piloerector muscles. These are the muscles that can make your hairs stand on end. In other animals like mice, cats, dogs; making your hair stand on end is a good thing. If you're cold you trap more air next to your body and it thermally insulates you. If you're scared it makes you look much bigger. If you look much bigger then you're scary to other animals and they're less likely to attack you. We're obviously derived and related to all these other mammals that do this but have less hair. It doesn't serve a useful purpose in us, it's just a sort of evolutionary vestige.