Anxiety, which can also refer to a syndrome, is a mood response to an internal threat that could be the expectation of an external threat. It is an emotional response that is out of proportion with any genuine threat found in the environment. Furthermore, instead of being directed towards a current stimulus, anxiety is associated with the anticipation of a future problem. Anxiety differs from fear in that fea r is a response to an immediate external, genuine threat. Physiologically, the symptoms of anxiety accord to those for fear, including nausea, light-headedness, shortness of breath, and increased heart rate--the typical fight-or-flight response; yet when there is no genuine danger in the environment, these symptoms can be extremely aversive and stressful for the individual. Although personal distress can lead to functional impairment, the diagnostic criteria for the syndrome of anxiety disorders only requires that the individual experience grave personal distress. There are several diagnostic categories for anxiety disorders: panic disorder with or without agoraphobia, agoraphobia with panic disorder, obsessive-compulsive disorder ( OCD), specific phobia, social phobia, posttraumatic stress disorder (PTSD), acute stress disorder (ASD), and generalized anxiety disorder.

Anxiety disorders, with a prevalence rate of 17 percent, represent the most common type of disorders. Regarding this prevalence rate, however, it must be noted that only about 25 percent of individuals suffering from a possible anxiety disorder seek treatment.

Specific and social phobias are the most common types of anxiety disorders, and generalized anxiety disorder and agoraphobia without panic disorder both have a prevalence rate of 3 percent. OCD has a prevalence rate of 2 percent and panic disorder has a point prevalence rate of 2 percent. Anxiety disorders are highly comorbid with each other and with mood disorders. All types of anxiety disorders except OCD are more likely to be found among women than men, and among the young than the elderly. Cross-cultural differences seem to affect the focus or content of typical anxiety complaints. From a historical perspective, individuals with anxiety disorders have not recei ved much attention from the medical community, since anxiety disorders seldom require institutionalization. Sigmund Freud and proponents of his theories, which focus on mental conflicts and innate biological impulses, were among the first to describe and classify anxiety disorders. According to Freud, anxiety has a very adaptive purpose of signaling to an individual that he or she is about to engage in an act previously associated with punishment and disapproval. This signal anxiety leads to ego defense mechanisms such as repression, which prevents the individual from recognizing consciously the forbidden impulse and from acting on the impulse, thus resulting in conformity to societa l rules and a decrease in anxiety. Nevertheless, if the level of pathological anxiety is too intense, extreme, or overwhelming, this could result in additional defense mechanisms that result in such disorders as compulsions and phobias. Regardless of the specific defense mechanism used, according to Freud, the underlying process remains the same for all types of anxiety disorders.