Goose bumps are caused by a reflex called piloerection. All mammals, including us, have skin covered with hair. Much of our body hair is quite fine -- sometimes barely visible. When the hair in the follicle lifts up (that's piloerection) it makes the hair follicle stand out. And that causes goose bumps, so named because they look like the skin of a plucked goose.

Piloerection puffs up furry mammals, making a cold animal warmer or a frightened animal more impressive. That's why when a kitten encounters a dog, it turns into a little hissing puffball.
Goose bumps are an automatic response, like sweating or increased heart rate. We can't easily control them.

Like other emotion-linked reflexes -- blushing, turning pale, butterflies in the stomach -- goose bumps are triggered by the limbic system of the brain. This governs primitive drives: sex, fear, rage, aggression and hunger. All the good stuff.

The limbic system is connected to two parts of the brain: the thalamus, which receives virtually all sensory input, and the hypothalamus. In humans, these parts of the brain allow emotional stimulation from music or the reading of poetry to cause goose bumps.

It seems music not only soothes the savage beast, it also makes it prickly. But why?

Poetry and music -- and even the scariest of movies -- don't make us cold, and aren't physically threatening. Why does our depth of human feeling make us respond like a frightened kitten?

Our automatic response to music or movies or emotion isn't driven by a physical prompt, but by a psychological one. What we see or hear or feel makes us vulnerable in a different -- but just as meaningful -- way.

Something that causes severe anxiety causes an adrenaline rush -- but so does something that causes intense pleasure or an emotional "welling up." Like the final scene in "Field of Dreams," when Kevin Costner asks his dad to play catch.

Our bodies often can't tell the difference between what's real and what's imagined. That's why you might get hungry watching a TV commercial that shows a hot, fresh pizza bubbling with cheese and pepperoni. It's also why you might get the wits scared out of you when the alien finally walks by in the movie "Signs" -- even though, logically, you know the theater is cozy and safe.

As we've become more civilized creatures, cerebral things have become more physically "real" to us. We don't need to puff up to fight rivals much anymore, but we do encounter and process many powerful emotional and psychological stimuli. That's our reality in the 21st century.

And the response varies from person to person.

"For some, being in a specific fearful situation causes goose bumps, but in another the same specific fearful situation does not," explains the University of Kansas' Dr. David Pendergrass, who's written about goose bumps and related responses.

"The key to understanding the perception is previous experience. A young child gets goose bumps because he is in a poorly lit room with whistling winds and long shadows and his best friend telling him a story about someone getting killed by a ghost in this very room. But if you were in the same room and being told the same story, you would probably not have goose bumps. The young child does not have the same previous experiences as you."

Author Stephen King, this generation's master of horror, has probably given goose bumps to as many people as anyone. Last year the National Book Foundation presented him its annual medal for distinguished contribution to American letters. It was the first time an author of a popular genre like horror had received the award.

King recalled being called "a hack, a terrible writer, everything that is wrong with America.

"After 25 years of that, to get something like this ..." King said, getting overwhelmed, "I got goose bumps."