Defining the boundaries between extremes of normal behavior and psychopathology is a dilemma that pervades all psychiatry. For some very extreme conditions, such as Downs syndrome, diagnostic decisions are straightforward. Milder forms, by contrast, present problems when one attempts to define the point at which “caseness” begins. A few symptoms escape this definitional conundrum by virtue of their being deviant, regardless of their severity. This applies to symptoms such as delusional beliefs or hallucinations. In the case of anxiety, however, it is especially problematic to establish the limits between normal behavior and pathology because when mild, anxiety plays an adaptive role in human development, signaling that self-protective action is required to ensure safety. Because anxiety can be rated on a continuum, some inves-tigators suggest that extreme anxiety represents only a severe expression of the trait, rather than a distinct or pathological state. Distributions may consist of distinct entities, however. For example, some cases of mental retardation, as caused by neurological injury, represent a quantal departure from factors influencing normal variations in intelligence. By analogy, the fact that anxiety falls on a continuum of severity does not preclude the presence of qualitatively distinct disorders at any point in the distribution (Klein & Pine, 2001).

Anxiety may become symptomatic at any age when it prevents or limits developmentally appropriate adaptive behavior (Klein & Pine, 2001). However, anxiety about particular circumstances may develop at one or another developmental stage, based on the typical age-related experiences that occur during this stage. For example, anxiety about separation represents a normal aspect of development that is experienced by many young children. Similarly, in adolescence, particular questions arise concerning anxiety about social situations, given changes in the social milieu that are experienced as stressful by many adolescents. A useful rule of thumb for determining the diagnostic threshold is the person's ability to recover from anxiety and to remain anxiety-free when the provoking situation is absent. For example, it is not necessarily deviant for adolescents to respond with acute discomfort or anxiety when meeting a peer that they find attractive. Such reactions reach clinical levels, however, when adolescents are unable to recover from the anxiety (as manifested by recurrent doubts or ruminations about how they behave), or when adolescents avoid such encounters on a consistent basis. Similarly, clinical anxiety in this situation might be characterized by the development of concerns about future meetings with unfamiliar peers or even avoidance of activities that might require peer interactions. Therefore, an adolescent's lack of flexibility in affective adaptation is an important pathological indicator. In addition, the degree of distress and dysfunction influences diagnostic decisions; these vary with developmental stage, as well as with cultural and familial standards. When anxiety symptoms are developmentally inappropriate, subjective distress is relatively more informative. For example, separation anxiety is developmentally more congruent with early childhood than with adolescence. In brief, three clinical features impinge on the definition of pathological anxiety. Two of these, distress and dysfunction, vary in importance as a function of developmental stage. The third, symptomatic inflexibility, is diagnostically relevant regardless of age.

The ability to draw firm conclusions on the ideal criteria for disorders will remain limited so long as signs and symptoms are the exclusive basis for establishing the presence of psychiatric

disorders. Longitudinal research can provide some answers by identifying specific symptom patterns and thresholds that have long-term significance. In practice, however, such evidence has proved to be informative but rarely conclusive.

The past two decades have witnessed a great expansion in the study of anxiety disorders. An earlier emphasis on rating scales or interviews assessing multitudes of unrelated fears and worries has been replaced by an emphasis on the study of diagnostic groups that reflect explicit clinical criteria. Scale ratings can be grouped to generate overall scores of anxiety, or what has come to be called “internalizing” symptoms, such as in the widely used Child Behavior Checklist (CBCL; Achenbach, 1991), but as the evidence shows, scale ratings correspond poorly to clinical entities.

Difficulties in separating “normal” from “pathological” anxiety are clearly reflected in results from epidemiological studies, in which the prevalence of anxiety disorders changes markedly with relatively minor changes in the definition of impairment (reviewed Klein & Pine, 2001). However, adolescents with anxiety disorders who seek treatment typically suffer from markedly impairing anxiety, and there is little ambiguity about determining whether they have “normal” or abnormal levels of anxiety.

This challenge poses both practical and conceptual problems. The practical problem concerns the timing of treatment. Two mistakes are possible. An adolescent who needs treatment may fail to receive it if the threshold for diagnosing the disorder is set too high (“a false negative”). An adolescent whose anxiety reflects a reasonable response to adverse circumstances may receive unnecessary treatment (“a false positive”). The decision to treat versus not treat is linked to costs and benefits that inform decisions about each adolescent.

The conceptual problem concerns the need to provide a principled basis for distinguishing disorder from nondisorder beyond the current imperfect clinically based principles. In an ideal circumstance, these principles would be based on understandings of pathophysiology. Consistent with this perspective, some philosophers of medicine have attempted to provide objective, biological criteria for demarcating disorder (e.g., major depression) from distressing states that fall within the bounds of normal unhappiness (e.g., grief). Others have claimed that all ascriptions of disorder reflect nothing more than societally determined value judgments about undesirable states and behavior.

Merging these polarized views, Wakefield (1992) proposed a harmful dysfunction account of disorder, holding the position that disorder is a hybrid concept comprising a factual component and an evaluative component. The factual component specifies what is dysfunctional—a derangement in a psychobiological function—and the value component specifies the resultant harm—usally emotional suffering, social maladjustment, or both. Therefore, ascription of disorder requires that two interrelated criteria be met: a psychobiological mechanism is malfunc-tioning, and this underlying dysfunction results in suffering, maladaptation, or both.

Wakefield's criteria imply that a person may be characterized by internal dysfunction but not qualify as having a disorder because no resultant harm occurs. For example, some youngsters characterized by extreme shyness or behavioral inhibition may find niches for themselves that enable them to adapt without marked distress. Even though the dysfunction requirement is met, these children would not be considered disordered because their dysfunction does not result in suffering or maladaptation. Conversely, some youngsters, bullied by larger children for example, may experience chronic anxiety at school. But because their suffering does not arise from dysfunction in the psychobiological mechanisms for estimating threat, Wakefield's criteria would suggest that they not be diagnosed as disordered. Mechanisms for detecting threat work precisely as they are “designed” to work: the bullied youngsters experience chronic anxiety because they are continually exposed to threat, not because they have a mental disorder. It is important to note that this is only one definition of a “mental disorder.”

Wakefield's (1992) framework is not without its limitations. Attempts to elucidate a value-free perspective on function—especially when cast within an evolutionary framework (McNally, 2001a)—raises yet another set of thorny prob

lems. Nevertheless, the harmful dysfunction provides a useful model for posing questions regarding the distinction between normal psychological distress and its pathological variants.

In the following sections we will describe each of the anxiety disorders listed in the Diagnostic and Statistical Manual of Mental Disorders, 4th ed., (DSM-IV; American Psychiatric Association, 1994). For an accompanying list of signs and symptoms see Table 9.1.