**4.1 From trauma to acute stress disorder to posttraumatic stress disorder**

Not every individual who experiences a traumatic event will subsequently develop posttraumatic stress disorder. As the name implies, posttraumatic stress disorder results from an experience with a traumatic stressor. Some of the stressors that can cause the disorder include: natural disasters, combat, sexual assault, physical assault, abuse or neglect as a child, car accidents, surgery, and witnessing something life threatening happen to a loved one. A person can develop posttraumatic stress disorder from a single stressor or may encounter multiple traumatic situations. An individual who encounters multiple events may either develop the disorder after the first event and the subsequent events then exacerbate their symptoms or they may develop the disorder only after experiencing multiple traumatic events.

As previously discussed, an individual must experience their symptoms for at least a month in order to receive a posttraumatic stress disorder diagnosis (American Psychiatric Association, 2000). Individuals with symptoms lasting less than a month are given an acute stress disorder diagnosis. Although everyone who has posttraumatic stress disorder has also had acute stress disorder, not everyone who experiences acute stress disorder will go on to develop posttraumatic stress disorder. The diathesis-stress model helps explain why some individuals do not develop the disorder after a traumatic experience (Elwood et. al., 2009). This model refers to the interaction between a person's environment (the severity of the stressors that they encounter) and their biological and psychological predispositions, which can create vulnerability for developing the disorder. Those with high diathesis only require a minimal stressor in order to develop the disorder, whereas someone with no diathesis may never develop the disorder even when presented with an extreme stressor. The next section will present the psychological and biological theories on the characteristics that may act as a diathesis for developing posttraumatic stress disorder.
