• Determine the temporal stability of the disorder.

• Determine whether the disorder aggregates in families.

This appendix is organized according to each of the five phases. Wherever possible, we draw on studies of veterans or other groups with comparable traumatic exposures.

Defining a disorder begins by describing individual cases and then proceeds to the gathering of more data. In the 1970s, clinicians treating Vietnam veterans described two core symptom clusters: reexperiencing a trauma through flashbacks and other symptoms, and reacting to the reminders of the trauma with avoidance (avoiding thoughts, feelings, or conversations about the traumatic event or situations that remind one of it) and emotional numbness (diminished responsiveness to the external world, feeling of detachment from other people, or having a marked inability to feel emotions, such as intimacy and tenderness). A third core symptom cluster, hyperarousal, was added in 1987 with publication of a revision of DSM-III, DSM-III-R, because high levels of arousal had been associated with PTSD in studies during the intervening years (Brewin 2003).

Several additional lines of evidence help to characterize PTSD’s core clinical features in the first phase of validating the disorder. The first draws from statistical studies that use factor analysis to explore whether PTSD symptom clusters cohere. The questions for research are, Do the symptom clusters (re-experiencing, numbing and avoidance, and hyperarousal) cohere (occur together)? Do they correspond to underlying biologic or psychologic processes? Asmundson et al. (2004) examined the results of numerous factor analysis studies and concluded that many of them support PTSD as having four core symptom clusters instead of three: re-experiencing and hyperarousal were indeed separate symptom clusters, but avoidance and numbing actually represented two separate ones instead of a single cluster, as defined in DSM-IV.

The second line of evidence draws from epidemiologic research suggesting that PTSD’s core clinical features have been found to be consistent among the diverse populations in which it has been studied.