Learning and unlearning fear: a clinical and evolutionary perspective.

Many fears, phobias and rituals seem to arise from prepared phylogenetic mechanisms which favor old over new evolutionary dangers and affect the rules of aversive learning which govern the acquisition of fear. Recent developments in several forms of aversive learning (sensitization, conditioning, extinction, observational learning) can improve them as paradigms of the acquisition, spread and maintenance of normal and clinical fears. The most reliable treatment for phobias and rituals is exposure, whose effects closely parallel the habituation of normal defensive responses and the extinction of conditioned fear and avoidance in animals. Habituation during exposure is usually slow and step by step, and generalizes little, but once attained tends to endure. Conditioned fear extinction and fear habituation have similar courses and may depend on similar neural processes. To be reduced, avoidance has to be prevented or the safety intervals that it heralds must be given up. Some phobias may result less from enhanced acquisition than from insufficient exposure to attain habituation. Finally, the review discusses the limits of habituation and the instability of fear extinction in relation to the long-term efficacy of exposure therapy.