intensity. For that reason, DSM-IV does not permit the diagnosis of PTSD before the passage of at least a month after the traumatic event.

To establish temporal stability, the same disorder must be present in the same person at a second time in a longitudinal assessment. For example, all the people in the index sample with a diagnosis might no longer report symptoms of that diagnosis at followup, but they might be replaced by an equal number of new cases in people who did not have the disorder at the index time. That is, the overall percentages with PTSD at index and followup could be the same, but the stability of the cases would be zero. Therefore, individual cases, rather than overall rates, should be examined because of individual fluctuations.

Temporal stability also requires that the condition not evolve into a different psychiatric disorder. It is important to evaluate a person for all the other disorders that are part of a differential diagnosis of PTSD, such as GAD and MDD, both at the index time and at followup. Meeting those two criteria requires that studies be longitudinal and use a broad-based assessment method that evaluates participants for PTSD and for other psychiatric disorders. However, the committee was unable to locate such studies for PTSD.

The fifth phase in validating a disorder is to determine whether it aggregates in families (Robins and Guze 1970). A key question is how a family history contributes to PTSD. Is it through genetic vulnerability, shared family environment, or both? Research in the 1980s suggested that PTSD occurred in families with other anxiety disorders and depression. It found that family members of veterans with PTSD had higher rates of GAD, alcoholism, and (to a lesser extent) MDD and other types of depressive illness (Davidson et al. 1989; Davidson et al. 1985). A family history of psychiatric illness might increase vulnerability to developing PTSD, but according to the conclusions of two recent meta-analyses, it plays a relatively small role (Brewin et al. 2000; Ozer et al. 2003).