The vast majority of community residents have experienced traumatic events. Only a small minority of victims (<10%), though, developed post-traumatic stress disorder (PTSD) (Kessler et al. 1995, 2005; Breslau et al. 1998b, 2004b), suggesting that victims vary in risk factors for the disorder (Breslau et al. 1991; Yehuda & McFarlane, 1995; Bowman & Yehuda, 2004). Risk factors modify the probability of PTSD following traumatic experiences, enhancing or diminishing the likelihood of the disorder. Two categories that modify the risk of PTSD have been delineated: trauma characteristics, chiefly trauma severity (Green et al. 1985; Kulka et al. 1990), and well-established predispositions, that is, prior psychiatric disorders and family history of disorders (for a meta-analysis, see Brewin et al. 2000). It is generally assumed that the influence of predispositions is inversely related to trauma severity: when traumas are extreme, predispositions are assumed to play a secondary etiological role (McNally, 2009). This assumption has been articulated in the context of a general stress–disease model (e.g. Rabkin & Struening, 1976). Although the assumption has a compelling intuitive appeal, it has never been directly tested.
