SOCIAL FACTORS Research on social factors and the risk for PTSD focuses primarily on (1) the nature of the trauma and the individual's level of exposure to it and (2) the availability of so- cial support following the trauma. Victims of trauma are more likely to develop PTSD when the trauma is more intense, life- threatening, and involves greater exposure (Neria & Galea, 2007). For example, victims of attempted rape are more likely to develop PTSD if the rape is completed, if they are physically in- jured during the assault, and if they perceive the sexual assault as life-threatening (Kilpatrick et al., 1989). Similarly, PTSD is more prevalent in Vietnam veterans who were wounded, who were in- volved in the deaths of noncombatants, or who witnessed atroci ties (Koenen et al., 2003; Oei, Lim, & Hennessy, 1990). A study of PTSD following September 11 terror attack found a greater prevalence among people who lived south of Canal Street, close to the World Trade Center (Galea et al., 2002). As with less severe stressors, social support after a trauma can play a crucial role in alleviating long-term psychological damage. A lack of social support is thought to have contributed to the high prevalence of PTSD found among Vietnam veterans (Oei et al., 1990). Rather than being praised as heroes, returning veterans often were treated with disdain. This made it difficult for many veterans to find meaning in their sacrifices and likely increased their risk for PTSD. People who had little social support also were more likely to develop PTSD following September 11 (Galea et al., 2002). A study of identical twins strongly supports the role of the environment in PTSD. Among 715 monozygotic (MZ) twin pairs who were discordant for military service in Southeast Asia during the Vietnam War era, the prevalence of PTSD was nine times higher for co-twins who served in Vietnam and experienced high levels of combat in comparison to their identical twins who did not serve (Goldberg et al., 1990). BIOLOGICAL FACTORS The same twin study also strongly points to biological factors in PTSD. In an analysis of more than 4,000 twin pairs, MZ twins had a higher concordance rate for exposure to combat than dizygotic (DZ) twins. Follow- ing exposure, identical twins also had higher concordance rates for PTSD symptoms than fraternal twins (True et al., 1993). Importantly, genetic contributions differed across symptoms. Genes contributed most strongly to arousal symptoms and least strongly to reexperiencing. Conversely, level of combat exposure predicted reexperiencing and avoidance but not arousal (True et al., 1993). Does exposure to trauma have biological consequences as well as biological causes? People with PTSD show differences in the functioning, and perhaps the structure, of the amygdala and hippocampus. These findings are consistent with the experience of heightened fear reactivity and intrusive memories (Kolassa & Elbert, 2007). Other evidence links PTSD with general psycho- physiological arousal, for example, an increased resting heart rate (Pole, 2007). This suggests that the sympathetic nervous system is aroused and the fear response is sensitized in PTSD. Does this mean that trauma damages the brain? A study of identical twins one Vietnam veteran with PTSD and his co- twin who neither served in Vietnam nor suffered from PTSD- found smaller than average hippocampus volume in both twins (Gilbertson et al., 2002). Twin research shows that preexisting differences account for IQ deficits that have been mistakenly attributed to brain damage due to trauma (Gilbertson et al., 2006). Differences between people with and without PTSD are correlations-correlations apparently due to preexisting differ- ences, not due to brain damage caused by trauma. PSYCHOLOGICAL FACTORS Some theories suggested that dis sociation is an unconscious defense that helps victims to cope with trauma (Oei et al., 1990). However, dissociation predicts more, not less PTSD (Ehlers, Mayou, & Bryant, 1998; Griffin. Resick, & Mechanic, 1997; Harvey et al., 1998). Among a sam- ple of Israeli war trauma victims, for example, more dissociation reported within one week following trauma predicted more severe PTSD six months later (Shalev et al., 1996). Preparedness, purpose, and the absence of blame can aid coping with trauma. Pilots cope more successfully with helicopter crashes if they have training than if they have none, underscoring the impor- tance of preparedness and control (Shalev, 1996). The value of pur- pose is supported by evidence that, despite greater physical suffering, political activists develop fewer psychological symptoms than non- activists following torture (Basoglu et al., 1997). On the other hand, negative appraisals-the rape victim who blames herself, or the driver who thinks he could have avoided an accident-are strongly tied to an increased risk for PTSD (Bonanno et al., 2011; Bryant & Guthrie, 2005; Halligan et al., 2003; McNally et al., 2003). Over time, victims of trauma must find a balance between gradually facing their painful emotions while not being over- whelmed by them. New York City college students had lower rates of PTSD following the September 11 terrorist attack if they were better at enhancing and suppressing emotional expression (Bonanno et al., 2004). This illustrates what psychologist Edna Foa, a leading PTSD researcher, calls emotional processing, which involves three key steps. First, victims must engage emotionally with their traumatic memories. Second, victims need to find a way to articulate and organize their chaotic experience. Third, victims must come to believe that, despite the trauma, the world is not a terrible place (Cahill & Foa, 2007; Foa & Street, 2001). This last step is similar to what other psychologists call mean- ing making eventually finding some value or reason for having endured trauma (Ehlers & Clark, 2000). Meaning making is very personal and may involve religion, a renewed appreciation for life, or public service. Importantly, the search for meaning is as- sociated with more PTSD symptoms, whereas finding meaning is linked to better adjustment (Park, 2010). Stephanie found mean- ing in her efforts to make others more aware of sexual assault. In the long run, many people actually report that trauma leads to growth (Tedeschi & McNally, 2011). Pesttraumatic growth, pos- itive changes resulting from trauma, is linked with less depression