the field of psychiatry to split with its nonscientific past and, through more systematic data-based studies, achieve an equal footing with the rest of medicine (Luhrmann 2000). “We believe that a valid classification is an essential step in science…One of the reasons that diagnostic classification has fallen into disrepute among some psychiatrists is that diagnostic schemes have been largely based on a priori principles rather than upon systematic studies” (Robins and Guze 1970). The a priori principles alluded to were psychoanalytic theories about causation of psychiatric illness. The work of Robins and Guze was elaborated on by Kendler (1980), Kendell (1989), and Kendell and Jablensky (2003), who distinguished between antecedent validators (precipitating factors), concurrent validators (psychologic tests), and predictive validators (diagnostic consistency over time).

Applying the five phases of Robins and Guze, this section examines the evidence that PTSD is a valid disorder (as defined below). It is not an exhaustive review but illustrates the complex nature of diagnostic validation, a process that often takes years or decades. The process is always receptive to new empirical evidence in shaping the outcome, the diagnostic criteria themselves. The five phases set a high bar for any diagnosis in medicine, and few existing diagnoses, whether in psychiatry or in other fields, would meet all of them. Therefore, fulfilling all the phases should not be seen as an absolute requirement for inclusion of a disorder in a classification scheme, such as DSM.

At the outset, it is important to emphasize that validating PTSD as a disorder is not the same as making a PTSD diagnosis. Making a diagnosis is guided by knowledge of diagnostic criteria, to be sure, but it also requires clinical judgment in application of criteria: “the specific diagnostic criteria included in DSM-IV are meant to serve as guidelines to be informed by clinical judgment and are not meant to be used in a cookbook fashion” (APA 2000).

The five phases for establishing the validity of a disorder (Robins and Guze 1970) are as follows:

• Describe the core clinical features of the disorder.

• Differentiate the disorder from other disorders.

• Conduct laboratory studies.