Effects of deployment on mental health in modern military forces: A review of longitudinal studies

By Peter Nasveld , Cristina Cotea , Stephen Pullman and Eva Pietrzak In   Issue Volume 20 No. 3 .


Background. Earlier studies presenting evidence that operational deployment negatively affects mental health outcomes among military personnel and veterans generally have lacked conclusiveness, largely because of cross-sectional or retrospective design.

Purpose. To review longitudinal studies investigating mental health outcomes of military personnel deployed in recent conflicts.

Methods. MEDLINE database was searched using relevant keywords and MESH terms. The US Millennium Cohort study website was used to obtain the list of relevant publications. Only prospective longitudinal cohort studies investigating mental health outcomes in deployed post Vietnam era military or veteran populations of developed countries were included.

Results. Eighteen studies fulfilled the inclusion criteria. Adverse effects included the increased incidence of post-deployment PTSD and depression. Individuals with the lowest functional scores and those exposed to previous traumatic assault were particularly vulnerable to a new onset of PTSD after combat exposure. Factors influencing the incidence of post-deployment PTSD included depression symptoms present during deployment, the presence of stress reaction during combat exposure and reception of associated frontline treatment, and the number of negative life events experienced after the traumatic event. More mental health problems were reported in soldiers returning from Iraq on the second screening conducted several months after their return, compared  with the first screening immediately upon their return. Some mental health symptoms (anxiety and depression) improved between deployments, while others (PTSD and panic attacks) did not improve.

CONCLUSION. The results indicate that combat exposure, not deployment in general , had an adverse effect on mental health.

Mental health indicators in personnel who were deployed but not exposed to combat were often better  than those in non-deployed personnel. Health outcomes and health needs were affected both by individual characteristics and post-deployment life events and these changed over time.

KEYWORDS: Military personnel, veterans, deployment, longitudinal study, mental health


Negative effects of deployment on the mental health of Gulf War veterans have been identified in many studies. Systematic reviews of cross-sectional studies presented good evidence of an increased frequency of self-reported symptoms of post-traumatic stress disorder (PTSD) and other common mental disorders 1. Similar observations of negative health outcomes, including mental health, were found in a large sample of Australian veterans of the era 2.

Australian Gulf War  veterans were at greater risk of developing post-Gulf War anxiety disorders including post-traumatic stress disorder, affective disorders and substance use disorders compared to non-deployed military personnel of the era. The prevalence of such disorders remained elevated a decade after deployment. The current PTSD rate assessed by structured clinical interviews 10 years after deployment was 5.4% 3.

Although there were several longitudinal studies of varied quality made   of the military that attempted to establish the causal relationship between deployment and other military-specific factors and various aspects of mental and physical health, they were rarely performed prospectively on a large cohort. After the 1991 Gulf War, the US Department of Defence recognised the need to collect prospective exposure and health information that may be associated with the long-term health of service members. Additionally, with a changed environment after the September 11 terrorist attack and with the deployment of an unprecedented number of troops to Iraq and Afghanistan, any negative health outcomes will affect a large segment of the population for years to come, increase the health needs of veterans and have a significant effect on medical and disability costs. Therefore, the accurate assessment of the effects of deployment on health becomes paramount.

To address this need, the US Millennium Cohort study was established 4, with a goal to prospectively evaluate the long-term health of military service members and the potential influence of deployment and other military exposures on health.

Australian troops take part in conflicts in Iraq and Afghanistan and also play a significant peacekeeping role in the Pacific region. Any knowledge regarding the effects of military deployment and military specific exposures could therefore allow for better preparation for the ensuing consequences of deployment.

A systematic review of prospective longitudinal cohort studies performed in the military was undertaken to investigate the often raised question of whether military service, in particular operational deployment, results in a higher risk of chronic illness among military personnel and veterans. The current review article presents the findings on the effect of deployment on mental health outcomes.


The MEDLINE database was searched using relevant keywords and MESH terms for Military Personnel /veterans, longitudinal study and health outcomes. Additionally, the US Millennium Cohort study website was used to obtain the complete list of relevant publications on the subject. The search was performed in July 2010.

To be included in the present review, studies had to be of prospective longitudinal cohort design and investigate mental health outcomes in military populations and veterans serving in post Vietnam War conflicts. Retrospective longitudinal studies and longitudinal panel studies were excluded.

The references found were downloaded to an EndNote library and assessed for relevance, based on the examination of titles and abstracts. There were 248 titles recovered, 49 were marked for inclusion, and after full text examination, 18 studies fulfilled the inclusion criteria and are reviewed here.

The  quality of studies was assessed on criteria that included cohort size, sample selection, follow-up rate and duration, outcome and exposure measurement bias, type of analysis, clarity of the results and adjustment for confounders.


Eighteen studies investigated mental health outcomes. PTSD was the main investigated outcome in 8 studies 5-12 and depression or stress in 10 studies 13-22. Four studies resulted from the US Millennium Cohort study 6, 9, 10, 22 and 14 studies investigated other military populations.

The main results of included papers are presented in the text below, while  details of the studies are presented in Table 1.

Seven studies, including all of the US Millennium Cohort studies were of very good quality, eight studies were of good quality, and one each were of moderate and low quality (see Table 1).

Self-reported symptom measures of PTSD and depression, assessed using validated instruments,were used in all studies.


Millenium Cohort

Among military personnel recently deployed to Iraq and Afghanistan and who did not have PTSD at baseline, the new onset of self-reported PTSD symptoms increases threefold in deployed military personnel with combat exposures compared to those of non-deployed personnel 9. Combat exposure in all Millennium Cohort studies was defined by being personally exposed to witnessing  (i) violent death  (ii) physical abuse (torture, beating, rape), (iii) dead and/or decomposing bodies, (iv) maimed soldiers or civilians, and (v) prisoners of war or refugees. Interestingly, the new onset of PTSD was less frequent in deployed personnel without combat exposure than in non-deployed personnel, indicating that combat exposures, not the deployment itself, affected the onset of PTSD. In those with PTSD present at baseline, deployment did not affect the persistence of symptoms. About 2.4% of Millennium Cohort members had self reported symptoms of PTSD at baseline. These symptoms were present in only 40-50% of the individuals at second assessment, which implies resiliency or recovery among more than half of the affected population between baseline and follow-up.

Although prior knowledge of post-deployment harmful effects is very useful, an understanding of the characteristics that confer particular vulnerabilities or resilience to new onset PTSD  could be of utmost importance. The mechanism of resilience or vulnerability to PTSD symptoms in individuals following overwhelming stress is not well understood. Some have suggested that repeated exposure to traumatic events makes people more resilient, others argue that it makes them more vulnerable. Although victims of prior assault and those with a history of mental illness have been shown to exhibit less optimal levels of mental health and higher risk for PTSD after a stressful experience, epidemiologic studies of PTSD in military members to date have been based largely on retrospective data, rendering investigation of etiologic pathways of PTSD inconclusive. Two papers investigated the factors contributing to vulnerability to PTSD in the US Millennium Cohort 6, 10.

One study investigated the effect of previous assault on the rates of new onset of PTSD 10. Eligible participants who were deployed to Iraq and Afghanistan between baseline and follow up had no PTSD symptoms at baseline and reported combat exposure at follow up. Out of five thousand participants, 28% of the women and 9% of the men, reported a previous assault at baseline, mostly sexual for women and violent for men. The rates of new onset of PTSD for the assaulted versus non-assaulted groups were, respectively, 22% and 10% in women and 12% and 6% in men. Adjusting for baseline factors, the odds of new-onset PTSD symptoms was more than 2-fold higher in both women and men who reported an assault prior to deployment 10.

The next study investigated whether baseline functional health status, as measured by SF-36 (Short Form-36), predicts new onset of PTSD among deployed military personnel with combat exposure 6. When over five thousand participants (eligibility criteria as in the previous study) were stratified according to their functional health measured by SF-36  score, 7.3% had new onset PTSD. Individuals with the lowest (<15th percentile) baseline mental or physical component summary scores of SF-36 had two to three times the risk of new onset of PTSD compared with those with higher scores (15th – 85th percentile). Of those with new onset PTSD, over half (58%) of cases occurred among 15% of participants with the lowest SF-36 scores.

Other Military Studies

Other military studies have investigated mainly the factors influencing the incidence of post-deployment PTSD.

In a large cohort of U.S. soldiers on a peacekeeping mission to Kosovo, depression symptoms present during deployment were a predictor of post-deployment PTSD 5.  However, pre-deployment screening for common mental disorders had a low predictability and would not have reduced subsequent morbidity or predicted PTSD in UK forces deployed to Iraq 8.

In Israeli veterans of the Lebanon War, the intensity of PTSD symptoms and mental health status assessed in three consecutive years after the war were worse in those who had a combat stress reaction (CSR) during the combat compared to those without CSR. However, the mental health status of the CSR casualties over the three years was stable or improved slightly 11.

In a similar group of veterans, twenty years after the war, traumatised soldiers who received frontline treatment for CSR had lower rates of posttraumatic and psychiatric symptoms, experienced less loneliness and reported better social functioning than similarly traumatised soldiers who did not receive frontline treatment 12.

There are also individual-based predictors of post-deployment PTSD. In a group of active duty US Army soldiers deployed to Iraq, neurocognitive performance prior to deployment was an independent predictor of the severity of PTSD symptoms 7.


Millennium Cohort

Male and female US service members who deployed and reported combat exposures were at an increased risk for depression compared with non-deployed service members, after adjustment for baseline PTSD symptoms and other potentially confounding variables 22.  Conversely, men and women who deployed and did not report combat exposures were at a lower risk for depression than non-deployed men and women. Thus, it is combat exposure not the deployment itself that is a risk factor for new-onset depression among US service members. In the absence of combat exposure, outcomes may be affected by selective deployment of service members who are at decreased risk for the development of depression in comparison with non-deployed men and women. The implications of these findings are that post-deployment screening for depression should be focused on US service members exposed to combat.

 Other Military Studies

In a large population of US soldiers who were screened for common mental problems directly after returning from Iraq and then 6 months later, more mental health concerns were reported at the second assessment 19. Based on the combined screening, about 20% of active personnel and 42% of reserve component soldiers screened required mental health treatment. Reported concerns included PTSD symptoms, interpersonal conflict and alcohol overuse. Although up to 60% of soldiers with PTSD symptoms identified on the first assessment improved by the second assessment, soldiers were still much more likely to report PTSD symptoms on the second assessment. This suggests either a failure of the first screening or a delayed onset of symptoms.

A smaller group of US soldiers, post-deployed from Iraq or Afghanistan and preparing to deploy again, were screened for PTSD, depression, anxiety, panic and alcohol overuse directly after return and then before re-deployment (about 7 months later). Post-deployment rates for all mental health measures combined were under 9%, with most around 5%. Levels of reported depression, anxietyand alcohol use decreased significantly between screenings one and two, but levels of reported PTSD and panic symptoms did not change. Results indicated that symptoms of panic and PTSD are less likely to spontaneously remit than other mental symptoms 15.

Two studies investigated the mental health of the UK Air Assault Brigade who were deployed to Afghanistan and Iraq with surprising results.

Those who completed questionnaires on arrival in Afghanistan and then on departure about 4 months later reported no significant change to mental health or alcohol use at the end of deployment compared with pre-deployment 14. Those screened before deployment and after about 4 months of service in Iraq reported slight but significant relative improvement in mental health 16. The result may  reflect  a limitation of these two studies, which were small and measured short-term effects, and participants were not stratified according to combat exposure. However, it is also possible that the lack of visible negative effects may be related to shorter deployment durations adopted by British forces as compared  with the US military 23, 24.

In a small cohort of Croatian soldiers assessed during the war and four times in the 10 years after the war , depressive and psychosomatic symptoms show different levels and trajectories. The level of psychosomatic complaints in soldiers was high during the war and increased steadily over time. Depressive symptoms were relatively low during the war but increased just afterwards 20.

For New Zealand peacekeepers deployed to low conflict zones, pre-deployment and follow-up post-deployment stages appear to be the most stressful periods of the deployment, with highest levels of anxiety and psychological distress seen at pre-deployment and 6 months after return 17.

Among Swedish peacekeepers serving during a low-intensity conflict, those that experienced traumatic events in Bosnia, as well as stressful life events post-deployment, reported the poorest mental health, with post-deployment stressors making the strongest contribution to poor mental health after one year 18.

A study of US soldiers undergoing the combat medic training, assessed at the beginning and the end of training 3 months later, found an increase in self-reported symptoms of depression, anxiety or suicidal ideation 21. Markers of distress increased as possible combat deployment became more imminent. Although this study was large, it investigated only short term effects and the further trajectory of these symptoms is unknown.


The cross-sectional studies on the prevalence of PTSD reported different rates in the military personnel from different countries or from different military forces. For example, in the US, PTSD rates among veterans of the US Persian Gulf War and the current conflict in the Middle East  varied between 2% and 17% 25. In contrast, PTSD rates among British veterans were generally lower and were less varied , about 3-6% of returning UK Iraq War veterans 25. PTSD rates found in different UK studies were 2.5% among a random sample of veterans from all branches deployed after 1999 26, 4.8% for regular UK army personnel  27, and 4% for regulars and 6% for army reservists  deployed to Iraq in 2003 28

In Australia, the prevalence of PTSD among younger veterans was  closer to that reported in veterans from the UK rather than the US. In Australian Gulf War  veterans, the rate of PTSD assessed a decade after deployment was 5.4% 3. The estimates of PTSD in ground forces of the ADF serving in Iraq and Afghanistan are as yet unpublished, but in Royal Australian Navy sailors deployed to the Middle Eastern Area of Operations between 2001 and 2005, the rate of PTSD was 1.4% in total 29. Usually, PTSD rates reported for military personnel deployed with the Navy and AF were lower than for the ground troops.

There are distinct benefits of prospective longitudinal cohort studies over cross-sectional and retrospective studies. Prospective longitudinal studies can distinguish between short-term and long-term phenomena, can contribute to establishing causative associations between exposure and disease, and minimise recall and selection biases that are often influenced by exposure and/or disease.

Longitudinal studies, such as the US Millennium  Cohort, demonstrate that it was combat exposure, not deployment in general, that has adverse effects on health 9.  Therefore, higher rates of PTSD are to be expected among troops with the greatest combat exposure (i.e. ground troops vs. Navy or Air Force, US troops versus Australian). The dose response between combat exposure and PTSD is not linear, but  a relationship between the amount and intensity of combat exposure and PTSD prevalence has been indicated previously in cross-sectional studies 30.

Studies from the US Millennium Cohort demonstrate conclusively that previous life events and health factors may constitute risk factors for the development of combat-related PTSD. Non-military trauma such as sexual or violent assault is a risk factor for newly reported PTSD and it appears to confer increased vulnerability for the development of PTSD symptoms 10. Low mental or physical health status before combat exposure significantly increases the risk of symptoms after deployment and a small proportion of individuals account for the majority of new cases of PTSD. The practical implications of these findings are that the more vulnerable members of a population could be identified by their health or life experience status and interventions and preventive measures could be focused on this group.

A very large US study demonstrated the importance of longitudinal screening for post-deployment mental health problems 19. Although the majority of soldiers with PTSD symptoms identified on first assessment improved by the second assessment, still more mental health concerns were reported at the second assessment 6 months after deployment than directly after returning from Iraq. This suggests either a failure of the first screening or a delayed onset of symptoms, and confirms the importance of an effective mental health screening policy. These findings are in agreement with a study of Australian Gulf War veterans, which mapped the temporal progression and peak prevalence of the most common psychological disorders across each year of the post-Gulf War period 31. Psychological disorder rates peaked in the first 2 years, with alcohol use disorders the most likely to appear first. In veterans with two or more disorders, anxiety disorders and alcohol disorders tended to appear before affective disorders. The changing trajectory of mental health problems after deployment or between deployments has been confirmed by most of the other military studies included in this review. These studies support findings that mental health problems and needs change in time and may increase with the accumulation of stressful events in post-deployment life. There is also a suggestion that improved screening and timely medical intervention may have beneficial effects.

Limitations of the Review

With few notable exceptions, which included all of the US Millennium Cohort studies, longitudinal studies of mental health in the military were limited  either by an insufficient sample size or by  investigations of short term health outcomes, making it difficult to draw definitive conclusions from these studies.


The results and conclusions drawn from the US Millennium Cohort studies represent the best level of evidence in the military context that presently can be obtained  from epidemiological observational trials.  The key finding from these studies was that it was combat exposure, not deployment in general ,that had adverse effects on health.

Another finding was that the mental and physical health indicators in deployed personnel were often better than those in non-deployed personnel, probably reflecting a selection of healthier individuals for deployment, while health outcomes and health needs change over time and are affected by individual characteristics and post-deployment life events.

As direct generalisation of results from the US Millennium Cohort and other studies are limited by differences in populations and different terms of deployment, longitudinal health surveillance of a large, representative sample of Australian Defence  Forces should be considered.


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This review was undertaken on behalf of the Longitudinal Health Surveillance Program, UQ Node, CMVH. Conflict of interest: The authors declare no conflict of interest