Interesting article from Newsweek:

The evolutionary primacy of the brain's fear circuitry makes it more powerful than the brain's reasoning faculties. The amygdala sprouts a profusion of connections to higher brain regions -- neurons that carry one-way traffic from amygdala to neocortex. Few connections run from the cortex to the amygdala, however. That allows the amygdala to override the products of the logical, thoughtful cortex, but not vice versa. So although it is sometimes possible to think yourself out of fear ("I know that dark shape in the alley is just a trash can"), it takes great effort and persistence. Instead, fear tends to overrule reason, as the amygdala hobbles our logic and reasoning circuits. That makes fear "far, far more powerful than reason," says neurobiologist Michael Fanselow of the University of California, Los Angeles. "It evolved as a mechanism to protect us from life-threatening situations, and from an evolutionary standpoint there's nothing more important than that."

I've already written about this sort of thing.