Many, many generations ago, when humans were much hairier, goose bumps served a useful function: They helped our ancestors’ hair stand on end in order to make them appear larger in threatening situations (the same way a cat bristles its fur when it’s scared). Today, we no longer need to stave off predators in this way—and without all the extra hair, our goose bumps are much more visible—but the response remains, which explains why goose bumps show up not only when you’re cold, but also when you’re afraid.
