The model is explained by posing a hypothetical example. The example involves two groups of individuals--police officers and emergency medical technicians--who are exposed to a traumatic event. The task is to assess the differences in vulnerability to the development of PTSD between the two groups. The first step is the development of measurement criteria for the traumatic event and for the formation of PTSD symptoms. Instruments are currently available that can do this. The next step is to separate the proposed regression model into four components. These are the "impact" with which trauma affects each group; the "exposure" each group has to the trauma; the "interaction" of vulnerability and exposure; and the "residual" vulnerability that is not associated with the current traumatic event. A score is developed for each component to show that police officers are more psychologically vulnerable to trauma than emergency medical technicians. This hypothetical analysis provides a method for formally isolating the effect of an event on psychological distress. The final section of the chapter identifies some of the determinants of vulnerability. These include individual factors such as biological markers and environmental factors such as social support, which can ameliorate the effects of traumatic stress. Lifetime experience and the socialization process can also influence the perception of vulnerability. Part of the shattering effect of trauma is the psychological disruption of an individual's perception of invulnerability. 24 references