After the Gulf War, veterans from three developed countries, the United States (Iowa), Australia, and Canada, displayed a similar prevalence of PTSD (Black et al. 2004; Goss Gilroy Inc. 1998; Ikin et al. 2004; McKenzie et al. 2004). The consistency of PTSD in diverse populations is also supported by studies that compared PTSD prevalence in developed and undeveloped countries. People in undeveloped countries and non-Western cultures might be expected, a priori, to have different ways of manifesting distress from trauma exposure (Department of Health and Human Services 2001), but PTSD has been documented worldwide in postconflict settings, including Algeria and Palestinian territories (de Jong et al. 2003), whether after war or after mass violence. In one study, cross-national similarities were found among civilians in the United States and Kenya after the same type of traumatic event, a terrorist bombing (North et al. 2005). That study evaluated people who several months earlier had been directly exposed to a terrorist bombing and also measured PTSD with uniformity of assessment (a fully structured psychiatric interview, the Diagnostic Interview Schedule, with adjustments for cultural differences in expression). The study found similar bombing-related PTSD prevalence among survivors in the two countries and similar prevalence in the two sexes: some 30% of men and 50% of the women from both the United States and Kenya had PTSD related to the terrorist bombing. Those studies of veterans from different countries and numerous other populations and traumatic events support the concept of PTSD as a distinct disorder.
A valid disorder is one that is distinct from others with similar symptoms, rather than merely another manifestation of them. The differentiation phase is difficult to apply to PTSD because it shares many symptoms with other psychiatric disorders, notably other types of anxiety disorders and major depressive disorder. Another difficulty is that a patient who has PTSD is likely also to have at least one other psychiatric disorder (a phenomenon known as comorbidity). Comorbidity, by itself,