The DSM-IV-TR criteria for ASD and PTSD are shown in Table 1 and Table 2, respectively. Table 4 compares the specific criteria used in making these diagnoses. For both ASD and PTSD, essential features are exposure to a traumatic event that need not be outside the normal range of human experience but that arouses "intense fear, helplessness, or horror"(DSM-IV-TR, p. 463), followed by development of characteristic symptoms. Exposure can occur through direct experience or through witnessing or learning about a traumatic event that caused "actual or threatened death,""serious injury," or "threat to the physical integrity" of oneself or others (DSM-IV-TR, p. 463). Both natural and human-made traumatic events have the potential to evoke these symptoms. Naturally occurring stressors include, for example, tornadoes, earthquakes, and medical illnesses. Human-made events include accidents, domestic and community violence, rape, assault, terrorism, and war. Some of these are singular events; others involve chronic or repeated exposure. In general, human-made events have been believed to cause more frequent and more persistent psychiatric symptoms and distress.

The criteria for ASD overlap substantially with but are not identical to those for PTSD (Table 4). Although core symptoms fall into characteristic symptom clusters for both diagnoses, ASD and PTSD differ in the numbers of symptoms from each cluster that are required to establish a diagnosis. For example, in addition to three or more dissociative symptoms and "marked avoidance of stimuli that arouse recollections of the trauma," the diagnosis of ASD requires at least one reexperiencing symptom as well as "marked" anxiety or increased arousal. On the other hand, for a diagnosis of PTSD to be made, DSM-IV-TR stipulates that there be at least one reexperiencing symptom, two arousal symptoms, and three avoidance/numbing symptoms and that these symptoms be temporally related to the stressor.Symptoms in the reexperiencing cluster include "recurrent and intrusive recollections" of the event, recurrent distressing trauma-related dreams, acting or feeling as if the event were reoccurring, "intense psychological distress" with exposure to trauma cues, and physiological reactivity to traumatic cues (DSM-IV-TR, p. 464). Within the avoidance/numbing cluster, purposeful actions as well as unconscious mechanisms may be present and may include efforts to avoid trauma-related thoughts, feelings, or conversations; efforts to avoid activities, places, or people reminiscent of the trauma; inability to recall important aspects of the trauma; greatly decreased "interest or participation in previously enjoyed activities"; feeling detached or estranged; restricted range of affect; and a "sense of a foreshortened future" (DSM-IV-TR, p. 464). Increased arousal includes sleep disturbance, "irritability or outbursts of anger," difficulty concentrating, hypervigilance, and exaggerated startle response (DSM-IV-TR, p. 464), all of which are generalized arousal responses and are not precipitated by reminders of the stressor.

The two disorders also differ in the duration of the disturbance and its temporal relationship to the traumatic stressor. For ASD, the disturbance occurs within 4 weeks of the traumatic event and is from 2 days to 4 weeks in duration. To qualify for a diagnosis of PTSD, symptoms must be present for more than 1 month. If symptom duration is less than 3 months, acute PTSD is diagnosed, whereas chronic PTSD is diagnosed when symptoms persist for 3 months or longer. Although symptoms of PTSD usually begin within 3 months of exposure, DSM-IV-TR also allows for delayed onset with symptoms that appear months or even years after the event. Finally, for both ASD and PTSD, the severity of symptoms must be sufficient to cause "clinically significant distress" or impaired functioning (DSM-IV-TR, pp. 468, 472).

A number of additional features may be associated with PTSD. According to DSM-IV-TR, these features include somatic complaints, shame, despair, hopelessness, impaired affect modulation, social withdrawal, survivor guilt, anger, impulsive and self-destructive behavior, difficulties in interpersonal relationships, changed beliefs, and changed personality. Difficulty seeking and sustaining medical care has also been observed (285). Symptoms such as inappropriate guilt, shame, or hopelessness may be indicative of comorbid depression that requires separate intervention, and other symptoms, such as somatic complaints, may represent common phenomena that are associated with anxiety disorders but are not necessary for the diagnosis of either ASD or PTSD. Finally, symptoms of trauma-related dissociation are essential to the diagnosis of ASD but are not necessary for the diagnosis of PTSD. Nonetheless, a previous history of peritraumatic dissociation (and ASD) may be of clinical significance in patients with PTSD, as studies have demonstrated that such a history predicts greater severity and chronicity of PTSD (7, 286, 287).

The differential diagnosis of ASD and PTSD includes a broad range of psychiatric and physical diagnoses as well as normative responses to traumatic events. Individuals who are exposed to events that fulfill criterion A for ASD or PTSD often experience some transient symptoms that differ from those of ASD or PTSD only in their duration or in the associated level of dysfunction or distress. In some professions (e.g., military, firefighters, police, emergency medical personnel), exposure to criterion A events is inevitable. If symptoms do not meet the criteria for ASD or PTSD but are persistent or associated with dysfunction or distress, a V code diagnosis (e.g., V62.2, occupational problem) may be appropriate.

Establishing a differential diagnosis also requires that ASD be differentiated from PTSD. For a single discrete traumatic event, ASD and PTSD can be readily distinguished from one another based on the time that has passed since the trauma. However, for less discrete or reoccurring traumas such as repetitive domestic violence, the distinctions between ASD and PTSD may be less clear. Although no convention or consensus exists regarding the classification of recurrent symptoms (for more than 1 month) during the course of repetitive episodic trauma, it may be best to conceptualize this symptom presentation as PTSD rather than as recurrent episodes of ASD. Clearly, eliminating the source or threat of continued violence and injury is critical to ultimate resolution of posttraumatic symptoms, regardless of diagnostic classification. As noted earlier, beyond duration of symptoms, the major distinguishing feature between ASD and PTSD is the emphasis in the former on dissociative symptoms. Although persons with ASD often develop PTSD, this is not invariably true. PTSD may also occur in persons who manifest few or even no symptoms of ASD in the period immediately after trauma (6, 7, 9). In patients with subthreshold or full symptoms of PTSD for less than 1 month who do not experience dissociative symptoms sufficient to meet the DSM-IV-TR criteria for ASD, the illness would be best characterized as an adjustment disorder in DSM terms. Such patients would also meet the diagnostic criteria for acute stress reaction, as defined by ICD-10. The differential diagnosis also includes medical disorders as well as a number of other psychiatric disorders (Table 5).

The fact that many of these disorders occur comorbidly with ASD or PTSD further complicates diagnosis. For example, a substantial proportion of trauma-exposed veterans (20, 247), refugees (292), and civilians (12, 293) develop symptoms consistent with major depressive disorder. Mood disorders are also an established risk factor for the development of PTSD in newly exposed individuals (12, 14, 34). Symptoms such as insomnia, poor concentration, and diminished interest in activities may be present with ASD and PTSD as well as with major depression. In addition, the restricted affective range that may accompany the numbing of responses with PTSD may resemble the restricted affect seen in depressed patients. It is important to note that if the DSM-IV-TR criteria are met, a major depressive episode can be diagnosed in conjunction with ASD or PTSD.

Trauma-exposed populations and patients with PTSD frequently experience comorbid substance-related disorders (256, 257, 294–299). Patients with PTSD also manifest increased physical complaints (76–79, 300, 301)and comorbid medical conditions (302). Although DSM-IV excluded complicated or prolonged grief as an axis I diagnosis (because of a lack of empirical evidence regarding symptoms), some investigators have proposed criteria for a diagnosis of complicated grief disorder based on patterns of prolonged bereavement characterized by persistence, intensity, intrusive recollections or images of the death, preoccupation with the loss, and avoidance of reminders (303). Furthermore, there is evidence that these symptoms may be more distressing after an unnatural or violent death. Such symptoms overlap with both major depressive disorder and PTSD, but persons may acknowledge these symptoms without meeting the criteria for either diagnosis. Here, preoccupation with the suddenness, violence, or catastrophic aspects of traumatic loss may be independent from and may interfere with the normal bereavement process (304). Consensus criteria for "traumatic grief" have been developed; these criteria overlap with those of complicated grief but incorporate additional symptoms of distress related to cognitive reenactment of the death, terror, and avoidance of reminders (289). Once again, studies that address treatment for these phenomena distinct from treatment for PTSD or depression are presently lacking. Nonetheless, complicated or traumatic grief as well as bereavement must be considered in the differential diagnosis for persons who have experienced a traumatic loss.

Finally, since childhood trauma may be a common antecedent to the development of personality (particularly cluster B) disorders in adulthood, and associated features of personality disorders and PTSD overlap (e.g., difficulty with affect modulation, impulsivity, irritability, comorbid substance abuse), PTSD symptoms may be "masked" by an underlying personality disorder. Numerous reports describe childhood trauma in adults with borderline personality disorder, and other reports describe childhood trauma as a root cause of adult PTSD. However, the extent to which symptoms may be misattributed to either PTSD or a personality disorder has not been well studied. Therefore, personality disorders must be considered in the differential diagnosis either as the primary etiology for symptoms or as comorbid illnesses.

Exposure to a traumatic event, the essential element for development of ASD or PSTD, is a relatively common experience, although the specific rates of such experiences within a population sample will vary with the criteria used to define a potential trauma as well as with the sample characteristics and the interviewing method (e.g., telephone survey versus face-to-face interview, clinician versus lay interviewer, structured versus unstructured interview), as reviewed by Brewin and colleagues (222). For example, using DSM-III-R criteria, which required that the event be outside the range of normal human experience, researchers in the National Comorbidity Survey (4) assessed 5,877 individuals ages 15–54 years with the Diagnostic Interview Schedule (DIS) and the Composite International Diagnostic Interview, administered by experienced nonclinician interviewers. They found that more than one-half of the subjects had experienced a traumatic event during their lifetime, with most people having experienced more than one. Giaconia and colleagues (305) also used the DSM-III-R version of the DIS and found that by age 18 years, more than two-fifths of youths in a community sample had been exposed to an event that was severe enough to qualify for a diagnosis of PTSD. Using structured telephone interviews in a national sample of 4,008 adult women, Resnick and colleagues (306) found a lifetime rate of exposure to any type of traumatic event of 69%. Using the DSM-IV version of the DIS, Breslau and colleagues (5) examined trauma exposure and the diagnosis of PTSD in a telephoned community sample of 2,181 individuals in the Detroit area and found that the lifetime prevalence of trauma exposure was 89.6%. The most prevalent types of events were the sudden unexpected death of a close relative or friend (60.0%) or learning of trauma to a close relative or friend (62.4%).

Overall exposure to traumatic events may be somewhat greater in men than in women (4, 5), although the gender difference in the lifetime prevalence of such exposure is relatively small (60.7% for men versus 51.2% for women in the study of Kessler and colleagues [4], and 92.2% for men versus 87.1% for women in the study of Breslau and colleagues [5]). In addition, men and women differ in the types of events to which they are exposed. For example, in the National Comorbidity Survey, 0.7% of men versus 9.2% of women had a lifetime experience of being raped, whereas 19% of men but only 6.8% of women had been threatened with a weapon and 6.6% of men but no women had experienced combat (4). In the Detroit Area Survey of Trauma (5), a similar pattern was noted, with women being more likely than men to report rape (9.4% versus 1.1%) or other sexual assault (9.4% versus 2.8%) and men being more likely than women to report other types of assaultive violence, including being mugged or threatened with a weapon (34% versus 16.4%) and being shot or stabbed (8.2% versus 1.8%).

Exposure to traumatic events also varies with age, showing consistent declines with age across multiple studies. For example, Norris (307) found a strong trend for decreases in both past-year and lifetime exposure with increasing age in a nonrandom sample of 1,000 individuals from four cities in southeastern states. Bromet and colleagues (14) analyzed data from the National Comorbidity Survey and found that the risk of experiencing a traumatic event was greatest in the 15- to 24-year-old cohort and decreased in subsequent age cohorts. Similarly, Breslau and colleagues (5) found that in all classes of traumas studied, peak exposures to traumatic events occurred in persons ages 16–20 years, with subsequent declines in exposure rates with age.

The lifetime prevalence of ASD is unclear, but a number of community-based studies have examined the prevalence of PTSD. Here, too, the reported rates vary with the specific diagnostic criteria employed, the interviewing method, and the sample characteristics. For example, in a study of the data for 2,985 participants from a central North Carolina community who were assessed as part of the Epidemiologic Catchment Area (ECA) survey, Davidson and colleagues (242) found a lifetime prevalence for DSM-III PTSD of 1.3%. Helzer and colleagues (308) found a lifetime PTSD prevalence of 1% in the St. Louis ECA sample. Using DSM-III-R criteria, Kessler and colleagues (4) found an estimated lifetime prevalence of PTSD of 7.8% in the National Comorbidity Survey, whereas Giaconia and colleagues (305) found that more than 6% of youths in a community sample met the criteria for a lifetime diagnosis of PTSD.

The likelihood of developing PTSD on having been exposed to a traumatic event (i.e., the conditional risk of PTSD) varies widely with the specific experience. Overall in the Detroit Area Survey of Trauma, for example, 9.2% of trauma-exposed persons developed PTSD, but PTSD developed in about half of those who were raped or held captive, tortured, or kidnapped, compared to only 2.2% of those who learned of the rape, attack, or injury of a close relative (5). In the women studied by Resnick and colleagues (306), rates of PTSD were significantly greater in crime victims that in non-crime victims (25.8% versus 9.4%).

General population studies typically find a significantly higher lifetime prevalence of PTSD in women, with rates that are consistently about twice those seen in men (4, 5, 222, 242, 308). The absolute rates for a lifetime diagnosis of PTSD again vary with the definition and severity of the traumatic stressor. Using the DSM-III criteria as part of the ECA survey, Helzer and colleagues (308) found that 1.3% of women and 0.5% of men met the criteria for a lifetime diagnosis of PTSD, and Davidson and colleagues (242) found lifetime rates of PTSD of 1.8% in women and 0.9% in men. In contrast, using the DSM-III-R criteria in the National Comorbidity Survey, Kessler and colleagues (4) found a lifetime prevalence for PTSD of 10.4% in women and 5.0% in men, and Breslau and colleagues (5, 223), using the DSM-IV criteria, found the lifetime prevalence of PTSD to be 13.0% in women, compared to 6.2% in men. In terms of the relative likelihood of developing PTSD after having experienced a traumatic event, Kessler and colleagues (4) found a twofold increase in the conditional risk of PTSD in women, compared to men (20.4% versus 8.1%). These gender differences in rates of PTSD do not necessarily imply that women are more likely to develop PTSD, per se; the differences may be explained by other factors that increase risk for women (15), such as the greater likelihood of women's experiencing rape and other sexual assaults, which carry a high conditional risk of developing PTSD. In addition, since a history of mood disorder increases the subsequent risk of developing PTSD in response to a stressor (14), the greater prevalence of such disorders among women may influence their likelihood of developing PTSD. Furthermore, specific aspects of the traumatic event, such as fear, threat, surprise, and meaning, may influence the victim's response (309).

The literature provides inconsistent information on the relationship between age and the risk of developing PTSD. Breslau and colleagues (33), in a representative community sample in southeast Michigan, found no relationship between age and risk of PTSD. In the National Comorbidity Survey, Kessler and colleagues (4) found some variations in the lifetime prevalence of PTSD by birth cohort, but men had the highest rates in the 45- to 54-year-old cohort, whereas women had the highest rates in the 25- to 34-year-old cohort. In terms of the conditional risk of developing PTSD after adjustment for the type of trauma exposure, a subsequent analysis of the National Comorbidity Survey data also showed variations in risk with age among men but a greater risk for PTSD among women in younger age cohorts (14). Brewin and colleagues (222) found weak effects of age in a meta-analysis of risk factors for PTSD but suggested that the differences may reflect confounding factors.

The prevalence of exposure to traumatic events as well as the development of PTSD also varies across racial and ethnic groups, with high rates of exposure to violence among African Americans, American Indians, and Alaska Natives, compared to members of more economically advantaged groups (310, 311). For example, in one study, 82% of American Indians and Alaska Natives had been exposed to one traumatic event, and the prevalence of PTSD was 22% (4). American Indians have a rate of violent victimization that is more than twice the national average (312), whereas rates of PTSD among American Indians and Alaska Natives are about threefold higher than in the general population. An investigation of Northern Plains Indian youths in grades 8 through 11 found that 61% had been exposed to some kind of traumatic event (313). These adolescents were reported to have more trauma-related symptoms but not substantially higher rates of diagnosable PTSD (3%), compared to the general population (313). A study of a Southwestern American Indian communitiy found even higher rates of experience of one or more traumatic events but also noted a higher prevalence of lifetime PTSD in this community, compared with the general U.S. population (314).

Because members of some racial and ethnic groups are more likely to have lower socioeconomic status, live in an inner-city area, or be U.S. combat veterans (315), and because such status is associated with an increased likelihood of experiencing undesirable life events (316), some racial and ethnic groups are more likely to experience ASD and PTSD (4, 314). Among veterans, an increased likelihood of traumatic early experiences (310–312, 317) may contribute to the increased rates of PTSD seen in African Americans, Hispanics, and American Indian/Alaska Natives after combat-related trauma (247, 310).

Differences in the rates of previous exposure to traumas may account, in part, for differences observed in rates of PTSD among U.S. veterans of differing ethnic and racial backgrounds. However, greater war zone exposure to traumatic experiences among African Americans (315) and American Indians (318, 319) is likely to play a large role as well. In terms of racial differences in rates of PTSD among U.S. veterans, the National Vietnam Veterans Readjustment Study found that although 10% of U.S. soldiers in Vietnam were black and 85% were white, more African American (21%) than European American (14%) veterans experienced PTSD (247). In the American Indian Vietnam Veterans Project (319), evaluation of random samples of Vietnam combat veterans from three Northwestern Plains reservations and one Southwest reservation between 1992 and 1995 showed that approximately one-third of the Northern Plains (31%) and Southwestern (27%) American Indian participants had PTSD at the time of the study. Approximately one-half had experienced the disorder in their lifetime (57% and 45%, respectively). This rate was far in excess of rates of current PTSD observed in the European American or African American veterans (247).

Hispanics also have been found to be at higher risk for war-related PTSD than their European American counterparts (247). Because the risk for Hispanics was higher than that for black veterans, minority status must not be the only risk factor (320). Of the Hispanic subgroups, Puerto Rican veterans have been found to have a higher probability of experiencing PTSD than others with similar levels of war zone stressor exposure (321). Because these differences in prevalence were not explained by exposure to stressors or acculturation and were not accompanied by significant reductions in levels of functioning, it has been proposed that differences in symptom reporting may reflect features of expressive style rather than different levels of illness (320).

National variations in rates of PTSD development have been reported across populations exposed to traumatic events. For instance, less than 5% of hospitalized European survivors of unintentional injuries (e.g., motor vehicle crashes, job-related injuries) appear to develop PTSD (322, 323). However, between 10% and 40% of survivors of both intentional (e.g., injuries associated with human malice, such as physical assaults) and unintentional injuries treated within acute care settings in the United States, England, and Australia appear to develop symptoms consistent with the disorder (34, 117, 293, 324–328). The explanations for these different rates include methodological differences, cultural differences, and diagnostic accuracy (329).

The prevalence of PTSD in countries where war and disease are endemic is substantially higher and has been reported to range between 9.4% and 37% of the population. For example, Bleich and colleagues (330), in a telephone survey of a representative sample of 512 Israeli adults, found that after 19 months of ongoing terrorist attacks, 16.4% had been directly exposed to a terrorist attack, 37.3% had an exposed family member or friend, and 9.4% of the sample met the symptom criteria for PTSD. Sabin and colleagues (331) found similar rates in a cross-sectional survey of Mayan refugees living in Mexico, of whom 11.8% met the symptom criteria for PTSD, as measured by the Harvard Trauma Questionnaire and Hopkins Symptom Checklist-25, 20 years after fleeing the civil conflict in Guatemala. De Jong et al. (332) used the Composite International Diagnostic Interview to assess for PTSD in community populations of four postconflict low-income countries and found a prevalence rate of PTSD of 37.4% in Algeria, 28.4% in Cambodia, 15.8% in Ethiopia, and 17.8% in Gaza.

Treatment-seeking refugees may have even higher rates of PTSD, ranging from 55% to 90% (333). Studies have revealed alarming rates of PTSD in immigrant communities with a high degree of preimmigration exposure to potentially traumatic experiences (e.g., Asian Americans and Hispanic Americans). For example, in some samples, up to 70% of refugees from Vietnam, Cambodia, and Laos met the diagnostic criteria for PTSD, in contrast to prevalence rates of about 4% for the U.S. population as a whole (334).

Studies of Southeast Asian refugees receiving mental health care have uniformly found high rates of PTSD. One study found that 70% of the subjects met the diagnostic criteria for PTSD, with Mien from the highlands of Laos and Cambodians having the highest rates (333). Another mental health study of Southeast Asian refugees (Hmong, Laotian, Cambodian, and Vietnamese) in Minnesota found that 73% had major depression, 14% had PTSD, and 6% had anxiety or somatoform disorders (335). A random community sample of Cambodian adults revealed that 45% had PTSD, and 81% experienced five or more symptoms of PTSD (336). Similarly, 43% of parents recruited from a community of resettled Cambodian refugees in Massachusetts reported the death of between one and six of their children (337). Child loss was associated with an increased likelihood of health-related concerns, a variety of somatic symptoms, and culture-bound conditions of emotional distress such as deep worrying and sadness not visible to others (337). Finally, Kinzie et al. (338) found that nearly one-half of a sample of Cambodian adolescents who survived Pol Pot's concentration camps as children had PTSD approximately 10 years after this traumatic period. Thus, many Southeast Asian refugees are at risk for PTSD associated with the events they experienced before they immigrated to the United States (311). A large community sample of Southeast Asian refugees in the United States found that preimmigration and refugee camp experiences were significant predictors of psychological distress even 5 or more years after migration (339). In this study, significant subgroup differences were found: Cambodians reported the highest levels of distress, Laotians were next, then Vietnamese. While trauma treatments may be effective for persons from Western cultures, in some Southeast Asian populations, it may be contraindicated to attempt to identify and process traumatic experiences (229).

Central American immigrants to the United States may be at risk for PTSD as a result of their preimmigration exposure to war-related trauma (340), even though they are not recognized as political refugees (311). For example, a study of Los Angeles adults who were examined for symptoms of PTSD and depression found that one-half of the Central American participants reported symptoms that were consistent with a diagnosis of PTSD (341). In comparison with recent Mexican immigrants, a greater proportion of Central American refugees reported symptoms of PTSD (50% versus 25%) (341). In another study, 60% of adult Central American refugee patients received a diagnosis of PTSD (342). Central American immigrant children seeking care at refugee service centers also had high rates of PTSD (33%) (343). In a more recent study of a systematic sample of 638 adult Latino primary care patients living in Los Angeles, Eisenman and colleagues (344) found that 54% of the sample had experienced political violence before migration, and of these, 18% had symptoms of PTSD. Those who had experienced political violence had a 3.4-fold greater risk of meeting the criteria for a PTSD diagnosis, compared to those who had not experienced political violence.

Prospective studies suggest that symptomatic distress peaks in the days and weeks after a trauma, then gradually declines over the course of the year after injury (139). In the National Comorbidity Survey, symptoms also decreased most rapidly in the first 12 months after trauma exposure (4). However, approximately one-third of persons who developed PTSD had chronic symptoms that did not remit. Although this issue is not settled (309), rates of recovery from PTSD may vary by gender. Although gender differences in the duration of PTSD are in part explained by gender differences in the type of trauma experienced, Breslau and colleagues (5, 226) found a median time to remission of symptoms of 12 months in men and 48 months in women. However, studies of motor vehicle accident victims have shown initial rates of approximately 35%, decreasing nearly 50% by 12 months postaccident (34, 345).

The responses of traumatized patients fall on a continuum, and the natural course of ASD and PTSD may vary with personality and other individual characteristics. Some individuals are relatively resistant to developing posttraumatic symptoms or report interpersonal growth experiences as a result of their traumatic exposure (229, 346). For other individuals with PTSD, however, long-lasting personality change may occur (252, 347–349). Problems of impaired affect modulation; self-destructive and impulsive behavior; dissociative symptoms; somatic complaints; feelings of ineffectiveness, shame, and despair or hopelessness; feelings of being permanently damaged; a loss of previously supportive beliefs; hostility; social withdrawal; feeling constantly threatened and being in an alert status; and impaired relationships with others all portend personality change from the individual's previous characteristics.

Investigations have also shown symptoms of PTSD to be associated with functional impairment and diminished quality of life (115, 117, 122, 293, 327, 350–353). Across veteran (122), refugee (292), and injured civilian (117, 293, 327) populations, PTSD makes an independent contribution to diminished functioning and quality of life above and beyond the effects of comorbid medical conditions and injury severity. Posttraumatic stress is also coupled with a spectrum of physical health problems and medical disorders (103, 354, 355). These considerations make the treatment of PTSD important not just from the standpoint of individual suffering but also from the perspective of the potential societal costs associated with the disorder (273, 356).

Individuals who have been exposed to trauma may also be vulnerable to subsequent traumas and have an increased likelihood of developing PTSD with repeated traumatic experiences (32, 33, 223). In individuals with a first hospitalization for psychosis, a similar pattern was observed, with exposure to multiple traumatic events being associated with greater rates of PTSD than exposure to a single trauma (48). These findings suggest that in trauma-exposed individuals, interventions should include efforts to decrease the risk for subsequent exposures to traumatic events.