Typical phobias include a fear of certain animals, certain environments such as heights, darkness, or enclosed spaces, or bodily harm such as blood, injury, or illness. A very common type of phobia is social phobia, which is a fear of being publicly humiliated or embarrassed. Notice that all of these things could occasionally really be dangerous! A phobia goes beyond normal fear however. For example, it's normal to feel nervous or afraid when you walk past a yard with a large german shepherd barking at you. A phobia, on the other hand, is an irrational fear of something specific that significantly interferes with your daily life. Someone with a phobia of dogs might avoid social situations, or going outside at all, for fear of seeing a dog.

Scientists believe that we have a natural tendency to experience certain types of fears, such as those listed above. Phobias may be a misfiring of those natural fears that leads to dysfunctional behavior. That's why it's common for someone to have a phobia of blood, but uncommon for someone to have a phobia of books (or something else non-threatening).

Why do some people develop phobias and not others? Childhood experiences may be a factor, but they don't fully explain the development of phobias. For example, some people with a phobia of dogs never had a bad experience with dogs as children, and some people who were bitten by dogs as children never develop a phobia.

Other important factors seem to be temperament (very shy children are more likely to develop phobias), neurobiological abnormalities (such as abnormal levels of the neurotransmitters serotonin and dopamine), and genetics (close relatives of people with a phobia are more likely to have phobias themselves).

At the neurological level, excessively high levels of activity in the amygdala may be related to phobias. The amygdala is the area of the brain typically associated with fear responses. Some people with phobias have too much activity in this area and this could underly their irrational fear responses.