As the various studies reviewed above demonstrate, there is consistency across these studies in linking combat experiences to increased risk for PTSD. The overall association between the degree of combat exposure and PTSD was identified in the NVVRS [47] and was later confirmed in an examination of external evidence corroborating combat exposure [17]. Studies with service members in the Gulf War [85] and in the wars in Iraq and Afghanistan have also confirmed this relationship [29, 77]. For example, in the Millennium Cohort Study , deployed personnel who reported combat exposures had close to four times the risk of new-onset PTSD compared to those who did not report combat exposure [71], although this relationship is not necessarily universal across military occupational specialties [73].

This robust relationship between combat exposure and PTSD warrants a further examination of specific combat exposure variables. There are several different ways of categorizing combat exposure variables , without a consistent method or approach reported in the literature. One method is to assess specific individual experiences (e.g., Pietrzak et al.[62]). Another method categorizes a list of combat exposure variables into rational categories (e.g., [21, 83]). A third method uses exploratory factor analyses to categorize combat exposure variables (e.g., [41]). While factor analysis can be used to identify distinct categories, the factors may reflect how frequently events occur in addition to identifying meaningful relationships among the events. Regardless of the method selected, there is little consistency in terms of which combat experiences are the most toxic with the possible exception of exposure to atrocities.

Early work by Breslau and Davis [12] found a link between amount of combat experiences and PTSD in a sample of 69 Vietnam veteran psychiatric inpatients. Specifically, they found a link with the items “buddy killed in action,” “separated from unit,” “witnessed atrocities,” and “participated in atrocities,” but the individual items “under enemy fire,” “combat patrol/dangerous duty,” “surrounded by the enemy,” and “wounded” were not significantly correlated with PTSD diagnosis. It is also important to note that there was also a strong relationship between participation in atrocities and PTSD, even after controlling for overall combat exposure. That is, regardless of the number of combat stressors reported, participating in atrocities conferred a strong risk of receiving a diagnosis of PTSD (but not for other psychiatric disorders like panic disorder, major depression, or a manic episode). In a separate study of 85 psychiatric inpatients (84 of whom were Vietnam veterans), the association between atrocities and PTSD was confirmed [24].

The lack of association between being wounded and PTSD was further explored in a study of 90 veterans of the war in Croatia from 1991 to 1993 [16]. In this interview study, soldiers who sustained nondisabling injuries were more likely to report PTSD than were soldiers who sustained disabling injuries or active duty soldiers who were not injured. The authors note that both groups of injured soldiers received injuries from the same kinds of combat-related events.

Other studies have attempted to organize a wide range of combat-related events into discrete categories. In an examination of NVVRS data, King et al. [43] followed a rational approach to categorize four war zone stressors : (1) traditional combat (e.g., firing a weapon, seeing dead Americans), (2) atrocities-abusive violence (e.g., observing events that raise questions of morality, involvement in terrorizing civilians), (3) perceived threat (e.g., the individual thinking he/she would not survive the situation or were in danger of being wounded/killed), and (4) malevolent environment (which included a degree of subjective assessment regarding difficult living conditions, perceived helplessness, and futility). Clinical psychologists and graduate students were used to sort the list of combat-related events into the four categories. In terms of association with PTSD, malevolent environment appeared to be the strongest; traditional combat was the weakest.

Fontana and Rosenheck [21] also used data from the NVVRS and examined the association of different types of combat exposure with benefits and liabilities. In contrast to King et al. [43], Fontana and Rosenheck categorized five types of combat exposure : (1) fighting, (2) killing, (3) perceived threat to self, (4) death/injury of others, and (5) atrocities. These categorizations were developed based on a review of common content by the authors. Each of these categories were highly correlated with PTSD (rs = 0.35–0.46), and the relationship between combat exposure and PTSD was mediated by perceptions of benefits (patriotic beliefs, self-improvement, and solidarity with others) and liabilities associated with combat.

In a later structural equation analysis, Fontana and Rosenheck [22] explicitly discussed how the various exposure categories are so intercorrelated that it is difficult to identify unique contributions of any one factor to the risk of PTSD. By testing for mediation , however, they were able to identify some relationships between combat exposure and PTSD. For example, they found that battlefield conditions fully mediated the impact of fighting on PTSD. Indeed, insufficiency of resources had a strong, direct relationship to PTSD (much like “malevolent environment” described by [43]). Furthermore, killing was strongly related to PTSD, and exposure to atrocities did not have a significant and unique association to PTSD once other variables (such as killing) were accounted for in the model. The relationship between personal threat and PTSD was also suppressed when other combat exposure categories were entered in the model.

Subsequently, Wilk et al. [83] had military experts sort a list of 33 combat exposure items into Fontana and Rosenheck’s [21] five categories. In addition, Wilk and colleagues also added a category to reflect positive experiences associated with combat exposure. In this analysis, which focused on alcohol misuse (not PTSD) and adjusted for demographics and unit cohesion, all six combat exposure categories were correlated with alcohol misuse (when tested in separate models). When psychiatric problems (including PTSD) were controlled for, threat to oneself and witnessing atrocities remained significant correlates of alcohol misuse as measured by a two-item screen. These results are consistent with earlier work highlighting the importance of atrocities as a risk factor for post-deployment adjustment. In an integrated model testing all of the combat exposure categories simultaneously, personal threat was still correlated with alcohol misuse, in contrast to the PTSD analysis reported by Fontana and Rosenheck [22]. Using the same categorization as Wilk et al. [83], Adler et al. [3] found a significant positive correlation over time between these six combat exposure categories and PCL scores (again, when tested in separate models).

Although not specifically examining PTSD, Killgore et al. [41] studied the relationship between different types of combat experiences and risk-taking propensity among a sample of veterans from Operation Iraqi Freedom. In contrast to the categorization approach described above, these researchers factor analyzed the list of combat exposure items and identified seven categories: violent combat exposure, US human trauma exposure, having survived a close call, having a buddy killed/injured, having killed enemy, having killed friendly/nonhostile forces, and pride in mission. While these categories were labeled differently, they are essentially similar to those proposed by Fontana and Rosenheck [21], with the exception of Killgore and colleague’s delineation between US human trauma and buddy injured/killed. In contrast, King et al. [43] had fewer categories and combined fighting, killing, and exposure to death into one category.

Regardless of which model is used, the most consistent finding appears to be that atrocities is associated with negative mental health , but there is less consistency regarding any of the other categories (e.g., [21, 22, 41, 83]). Other studies have found some consistency in terms of the relationship between killing and PTSD. In a series of studies on the association of killing to PTSD symptoms, Maguen and colleagues have repeatedly found that after controlling for other combat exposures, killing accounts for a significant but small amount of variance in PTSD symptoms. This relationship was found using NVVRS interview data [53], surveys with Gulf War veterans [55], and post-deployment screening data with US veterans of the Iraq war [54]. Taken together with other studies (e.g., [12, 22]), there appears to be some convergence that combat-related killing is associated with PTSD, but the degree to which it is a principal driver of combat-related PTSD or the degree to which other variables mediate its impact remains unclear.

It is also important to acknowledge the way in which combat stressors are defined. While most of the studies cited in this section have assessed exposure to potentially traumatic events, studies have also indicated that the malevolent environment, or nontraumatic stressors associated with the deployment environment, is also an important risk factor [78]. In one study, for example, service members reported that these kinds of nontraumatic stressors were actually more stressful than combat experiences [28]. Interian, Kline, Janal, Glynn, and Losonczy [33] also found that nontraumatic stressors related to the home front were linked with higher levels of PTSD. Despite the undisputed significance of combat-related traumatic events in predicting PTSD, these nontraumatic stressors also represent an independent risk factor for the development of PTSD. Given that these nontraumatic stressors may be easier for organizations to address, they provide an important perspective on the risk of developing PTSD.

In summary, the majority of studies seem to demonstrate a complex relationship between combat experiences and PTSD. Rather than one particular type of experience predicting PTSD, it may be the overall environment and the way in which these events are interpreted that result in PTSD. Another possibility is that it is the sheer number of events, rather than the different categories, that account for PTSD. Few studies have examined the curvilinear nature of PTSD but, as Adler et al. [2] reported, there may be a point at which the relationship between combat events and PTSD rises exponentially. That is, there may be a tipping point or threshold linked to the sheer number of combat-related demands whereby the service member develops PTSD rather than a specific category of exposure that drives PTSD symptoms. Perhaps it is more useful, therefore, to examine moderators of the combat exposure-PTSD link rather than identify specific combat-related experiences that place service members at risk.

Several studies have examined variables that influence the combat exposure-PTSD link, through a direct association, mediation, or moderation. These variables include preexisting psychological problems [14, 34], demographic variables like junior rank and younger age [34, 85], as well as variables measured either during deployment or upon return home, such as lack of social support [34, 76], low levels of psychological hardiness [76], low use of problem-focused coping (e.g., [70]), hostile unit climate (e.g., [23]), and low morale [34]. Similarly, Jones et al. [37] found that greater unit cohesion, high morale, and good leadership were all associated with lower levels of PTSD among UK Armed Forces personnel surveyed during their deployment to Afghanistan. Furthermore, specific leader behaviors addressing operational stress (e.g., “encourages soldiers to seek help for stress-related problems,” “intervenes when a soldier displays stress reactions such as anxiety, depression or other behavioral health problem”) were associated with fewer PTSD symptoms [4]. This relationship held even after controlling for generally good leadership, rank, and combat exposure. Note that while there is also evidence that gender influences the combat exposure-PTSD relationship, examining the impact of gender is beyond the scope of this chapter (for reviews of gender and combat-related PTSD, see [39, 43, 85]).

In an analysis of NVVRS data, results of structural equation modeling demonstrated the importance of hardiness as a personal resource as well as social support during the homecoming period in predicting PTSD [44]. Postwar stressful life events also accounted for PTSD symptoms, primarily through the lack of support and hardiness. Interestingly, these variables served as mediators but not moderators of the combat exposure-PTSD relationship. Thus, the impact of combat exposure (traditional combat, atrocities-abusive violence, perceived threat, and malevolent environment) was not exacerbated by low levels of hardiness or lack of social support, but their impact on PTSD occurred indirectly through the absence of these resources. Those high in hardiness appeared to seek out support and do better in terms of PTSD.

Building on the results of the NVVRS data, Vogt et al. [80] studied US veterans returning from Iraq and Afghanistan to identify risk factors for PTSD symptoms. Risk pathways from King et al. [45] were replicated, including the importance of pre-deployment risk factors (childhood family functioning and stressors), warzone stressors (combat exposure, perceived threat, and concerns about relationship disruptions during deployment), and postwar variables (stressors and social support). In particular, the perception of threat mediated the relationship between combat exposure and PTSD symptoms. The importance of cognitive appraisal has been replicated in other studies as well (e.g., [57]).

The goal of the foregoing discussion has been to provide a sense of the breadth of risk factors associated with the development of combat-related PTSD and the kinds of methodological issues associated with identifying these risk factors. While not exhaustive, this discussion provides a basis from which to understand that there are preexisting risk factors, risk factors associated with the deployment context, and risk factors that are evident at post-deployment as well. Table 2.2 provides a summary of some of the risk factors discussed in this chapter.

As discussed previously, the specific combat event itself may be less critical than how the individual interprets that event. However, to date, the research has addressed the role of individual cognitive processing of events and has not examined the shared processing of events by military units. Bliese et al. [10] have discussed the importance of considering unit-level characteristics in understanding unit-by-unit differences in the relationship between combat exposure and PTSD. Just as the individual’s cognitive processing appears to influence the impact of events on their mental health, so too may the military unit’s processing as a whole affect the group’s adjustment.

There are different elements that may affect group-level processing, from shared perceptions of unit cohesion, level of training, and leadership to shared perceptions of loss, betrayal, and social support. More research is needed, however, to clarify how unit-level processing of events influences the relationship between combat experiences and PTSD. For example, it may be that consistency of perceptions across the unit is as predictive of adjustment as actual ratings of those perceptions. As Bliese and Britt [8] found in their analysis of survey data from US soldiers deployed to Haiti, while the overall group perception of leadership quality was a significant moderator of the relationship between work stressors and outcomes such as morale and depression, group consensus about the quality of unit leadership was also a significant moderator of that relationship. This finding demonstrates the potential power of the group’s perception to impact a range of military-related mental health variables . Kok et al. [46] have also emphasized that understanding the military context is critical for identifying underlying patterns of PTSD.

At this point, we have reviewed the topics of diagnosis, prevalence, risk factors, and moderators associated with PTSD. One variable that is important to consider across all four of these topics is sleep. In terms of diagnosis , sleep problems are included in the DSM-5 criteria for PTSD [7]. Specifically, having traumatic nightmares is one of the symptoms in the intrusion symptom domain, and sleep disturbance is one of the symptoms in the domain of alterations in arousal and reactivity. Although sleep problems are included as part of the symptom picture of PTSD, sleep problems themselves are a relatively frequent problem reported by service members (e.g., [2, 69]) and are also correlated with PTSD [59, 69, 75, 84].

Not only is there an association between sleep problems and PTSD, sleep problems may be a precursor to the development of combat-related PTSD. For example, pre-deployment sleep problems serve as predictors of post-deployment mental health symptomatology in service members, including PTSD [25, 86]. In addition, Wright et al. [87] found that sleep problems reported at 4 months post-deployment were a risk factor in the development of PTSD (and depression) symptoms 8 months later. In contrast, PTSD (and depression symptoms) did not predict the development of sleep problems over time, providing support for the premise that sleep problems could serve as an early warning indicator of mental health problems. Not only are sleep problems indicators of increased risk and chronicity of PTSD, but sleep problems may affect the efficacy of evidence-based therapies for PTSD as well (see [26] for review).

Identifying the role of sleep in the development of PTSD is important because it offers two key avenues for intervention. First, it is possible that addressing sleep problems early may reduce the risk of subsequent PTSD symptomatology, although to date no early interventions have demonstrated the efficacy of this approach. Second, reporting sleep problems may be less stigmatizing than reporting other more traditional mental health symptoms. Thus, evaluating sleep problems may be a way to identify service members who can benefit from care in a way that is more acceptable to service members. Understanding this larger context is important in order to provide treatment in a way that is consistent with the occupational culture of the military.

Understanding the dynamics of combat and PTSD within an occupational context is different from understanding how an individual victim of a potentially traumatic event develops PTSD. In the case of the individual victim, the traumatic event is typically unexpected, and the individual is not trained to respond. In the case of high-risk occupations, PTSD symptoms can be reconceptualized using an occupational health model [15].

The occupational health model of PTSD assumes that individuals in high-risk occupations are not passive victims of potentially traumatic events; these events are encountered as part of the occupation for which they are trained, and the individual may be an active participant in these events. As exemplified by Perrin et al. [61], experienced emergency rescue personnel reported fewer PTSD symptoms in the wake of responding to the World Trade Center disaster than did rescue volunteers. Professional experience, identity, group support, and training may be critical protective factors.

Indeed, training is an essential component of the occupational health model. In the case of the military, for example, service members who rated their training and deployment preparation highly reported fewer mental health problems following deployment to Iraq and Afghanistan [63]. Furthermore, Adler et al. [1] found that service members reported that their training “kicked in” when they were confronted with a combat-related event like a firefight. Even though they reported symptoms of PTSD, they did not report the subjective peri-traumatic response of feeling helpless, horrified, or afraid (a DSM-IV criterion for PTSD that was dropped by DSM-5).

In an occupational health approach, the context of the event and the presence of group support are also important. Combat-related events typically occur to cohesive teams that provide normative information about how to respond as well as essential support to each other. These teams also have a strong expectation that they will look out for one another and that leaders will also look out for their unit members. This group perspective provides an important source of strength and support.

In addition, PTSD symptoms may occur in a different pattern than those that occur in response to an individual trauma . For example, some symptoms, such as hypervigilance, may occur prior to a specific combat-related event in response to training. The occupational health model also suggests that some symptoms are adaptive combat-related skills that can be used to help service members adjust to combat and are not necessarily dysfunctional in a combat environment. For example, anger can help focus an individual’s response and enhance performance during a combat deployment, but it is not particularly helpful in terms of relationship building following deployment. When service members transition home, these symptoms can become problematic, interfere with adjustment to noncombat missions, and may even increase as the supportive structures change with less unit contact or after an individual leaves the military.

Overall, the occupational health model can account for the presence of psychological reactions prior to exposure to potentially traumatic events, widen the domain of reactions typically considered (such as anger, guilt, grief, risk-taking, and potential benefits associated with deployment), and suggest different potential symptom trajectories. The occupational health model also proposes a basis for understanding the importance of variables that can reduce the impact of combat-related events on service member mental health.

By conceptualizing symptoms as part of the occupational context, the occupational health model can also suggest new avenues of research. As mentioned previously, unit-based cognitive appraisal should be examined to understand the role of shared interpretations of events in the development of PTSD. In addition, cognitive appraisal training should be studied as a method for helping service members and their leaders manage the demands of combat (e.g., [80]). Or early intervention effort and as part of the US Army’s resilience training program [65].

These training techniques, however, should not just be evidence informed but should also be evidence based. While some post-deployment early intervention techniques have been validated (e.g., [2]), more research needs to be conducted on using evidence-based techniques within the military [49]. Not all techniques based on civilian research are necessarily a good fit for the military context and may need to be adapted. These adapted interventions should be designed to leverage strengths in military culture, including the tradition of taking care of one another and meeting training standards. By integrating these characteristics into an intervention package, evidence-based military mental health training programs can be designed and implemented to be optimally effective in reducing the link between combat exposure, PTSD, and other post-deployment mental health problems.

Given the devastating consequences of PTSD in service members returning from combat coupled with the impact it has on their families, it is important to capitalize on three emerging trends . First, the field should take a public health perspective in understanding PTSD. While emphasizing treatment is important, it is equally important to understand the role of early interventions in reducing symptomatology. Although early interventions may not be easily studied with individual victims of trauma, studies can be conducted with groups that are at high risk for exposure to occupationally related traumatic events. Service members, like other high-risk occupations in the emergency services sector, are uniquely prepared to place themselves in harm’s way. As such, studies can be conducted to validate prevention programs designed to reduce the negative impact of occupationally related traumatic stressors on individuals.