In contrast to fear and panic, the anxiety response pattern is a complex blend of unpleasant emotions and cognitions that is both more oriented to the future and much more diffuse than fear (Barlow, 1988 , 2002 ). But like fear, it has not only cognitive/subjective components but also physiological and behavioral components. At the cognitive/subjective level, anxiety involves negative mood, worry about possible future threats or danger, self-preoccupation, and a sense of being unable to predict the future threat or to control it if it occurs. At a physiological level, anxiety often creates a state of tension and chronic overarousal, which may reflect risk assessment and readiness for dealing with danger should it occur (“Something awful may happen, and I had better be ready for it if it does”). Although there is no activation of the fight-or-flight response as there is with fear, anxiety does prepare or prime a person for the fight-or-flight response should the anticipated danger occur. At a behavioral level, anxiety may create a strong tendency to avoid situations where danger might be encountered, but there is not the immediate behavioral urge to flee with anxiety as there is with fear (Barlow, 1988 , 2002 ). Support for the idea that anxiety is descriptively and functionally distinct from fear or panic comes both from complex statistical analyses of subjective reports of panic and anxiety and from a great deal of neurobiological evidence (e.g., Bouton, 2005 ; Bouton et al., 2001 ; Davis, 2006 ; Grillon, 2008 ).
The adaptive value of anxiety may be that it helps us plan and prepare for possible threat. In mild to moderate degrees, anxiety actually enhances learning and performance. For example, a mild amount of anxiety about how you are going to do on your next exam, or in your next tennis match, can actually be helpful. But although anxiety is often adaptive in mild or moderate degrees, it is maladaptive when it becomes chronic and severe, as we see in people diagnosed with anxiety disorders.
Although there are many threatening situations that provoke fear or anxiety unconditionally, many of our sources of fear and anxiety are learned. Years of human and nonhuman animal experimentation have established that the basic fear and anxiety response patterns are highly conditionable (e.g., Fanselow & Ponnusamy, 2008 ; Lipp, 2006 ). That is, previously neutral and novel stimuli (conditioned stimuli) that are repeatedly paired with, and reliably predict, frightening or unpleasant events such as various kinds of physical or psychological trauma (unconditioned stimulus) can acquire the capacity to elicit fear or anxiety themselves (conditioned response). Such conditioning is a completely normal and adaptive process that allows all of us to learn to anticipate upcoming frightening events if they are reliably preceded by a signal. Yet this normal and adaptive process can also lead in some cases to the development of clinically significant fears and anxieties, as we will see.
For example, a girl named Angela sometimes saw and heard her father physically abuse her mother in the evening. After this happened four or five times, Angela started to become anxious as soon as she heard her father’s car arrive in the driveway at the end of the day. In such situations a wide variety of initially neutral stimuli may accidentally come to serve as cues that something threatening and unpleasant is about to happen—and thereby come to elicit fear or anxiety themselves. Our thoughts and images can also serve as conditioned stimuli capable of eliciting the fear or anxiety response pattern.