I’ve been reading Head Strong, an excellent new book by Michael D. Matthews, a professor of engineering psychology at West Point. The book describes the history and future of military psychology, from the birth of intelligence testing during WWI to the next generation of immersive battlefield simulations.

Not surprisingly, the problem of Post-Traumatic Stress Disorder (PTSD) is a recurring theme, as Matthews discusses recent attempts by the Armed Forces to promote resilience. (“The military does a good job of teaching its soldiers to kill. But it does not do a good job of teaching them to cope with it,” he writes.) Matthews details the Comprehensive Soldier Fitness (CSF) program, based on the work of Martin Seligman, and the unintended consequences of creating weapons systems so effective that they “give the individual soldier the firepower of a traditional squad or platoon.” One potential downside of these new systems, Matthews argues, is that American soldiers will gain the ability to control a large territory by themselves, and thus end up isolated from their comrades. “Soldiers fight for their buddies, who traditionally they could literally reach out and touch,” he writes. While technology makes the dispersal of troops possible, Matthews suggests there will be no substitute for the “physical presence of others,” especially when soldiers are “placed in situations of mortal danger.”

Interesting stuff. But there was one data point in the book that I couldn’t stop thinking about, even though Matthews mentions it almost as an aside. While pointing out that PTSD rates vary widely between military units – the overall rate for deployed soldiers hovers between 10 and 25 percent – Matthews notes that “highly trained and specialized units including SEAL teams, Rangers, and other elite organizations” have proven far more resistant to the disorder. (Their PTSD rates are typically less than five percent.) What makes this statistic even more surprising is that these elite units tend to see frequent and intense combat – in objective terms, they have experienced the most trauma. And yet, they seem the least troubled by its aftermath.

Why are elite units so resilient? There are many variables at work here; PTSD is triggered by a multitude of risk factors. For starters, elite units tend to be better educated and in better physical condition, both of which are correlated with a reduced incidence of PTSD. Self-selection also plays a role: anyone tough enough to become a Ranger or Seal has learned how to handle stress and hardship.

Matthews, however, mentions a protective factor that is often overlooked, at least in popular discussions of PTSD: unit cohesion. According to Matthews, elite units are “highly cohesive"; the soldiers form close relationships, built out of their shared experiences. In Pentagon surveys, they are more likely to agree with statements such as “my unit is like family to me,” or “members of my unit understand me.”

A series of recent studies backs up Matthews’ argument, highlighting the protective effects of unit cohesion. One analysis of 705 Air Force medical personnel deployed as part of Operation Iraqi Freedom found a “significant linear interaction…such that greater cohesion was associated with lower levels of PTSD symptom severity.” When stress exposure was high, for instance, medics in the most cohesive units reported PTSD symptoms that were approximately 25 percent less severe, at least as measured by the military’s PTSD checklist. Another study of 4901 male personnel from the UK armed services (Royal Navy, Royal Marines, British Army and Royal Air Force) concluded that unit cohesion was associated with significantly lower levels of PTSD and other mental disorders, such as depression. The British scientists end their paper by stressing the importance of fostering unit cohesion among soldiers, given "that so many other factors which have a positive association with higher levels of mental health problems are un-modifiable (for example, family background and exposures on deployment)." When it comes to PTSD, cohesion isn't just an incredibly important variable - it's a variable the Armed Forces can influence.

The explanation for these results is straightforward: in the aftermath of a terrible life event, other people are the best medicine. It doesn’t matter if we’re being helped by another soldier or a loving spouse - it’s really hard to get over the trauma alone. According to a highly cited meta-analysis of the risk factors associated with PTSD, a lack of social support is incredibly dangerous for those dealing with an acute stressor. (Among military subjects, a lack of social support was the single most important risk factor; among civilians, it placed second.) Close relationships, in this sense, are the ultimate coping mechanism, allowing us to survive the worst parts of life.

In some instances, the presence of close relationships seems to matter more than the stressor itself. Consider a natural experiment that took place during World War II, when approximately 70,000 young Finnish children were evacuated to temporary foster homes in Sweden and Denmark. For the kids who stayed behind in Finland, life was certainly filled with moments of trauma and stress — there were regular air bombardments, severe food shortages and invasions by the Soviets and the Germans. Those kids sent away, however, experienced a different kind of stress. Their wartime experiences might have featured less actual war, but the lack of social support would prove, over time, to be even more dangerous. A 2009 study found that Finnish adults who had been sent away from their parents between 1939 and 1944 were nearly twice as likely to die from cardiovascular illness as those who had stayed at home. A follow-up study found that these temporary war orphans also showed higher levels of stress hormone, stress reactivity and depression, sixty years after they’d been separated from their families. Chronic stress sucks. But chronic stress in the absence of supportive relationships can be crippling.

Perhaps this is why soldiers in elite units are so resilient. When the Armed Forces take unit cohesion seriously, they turn out be remarkably good at it, able to create deep, emotional bonds among their members. Over time, these relationships become an essential part of how soldiers cope with the violence. While unit cohesion has traditionally been seen through the prism of combat performance – more cohesive units perform better in battle – it seems likely that the biggest benefits of cohesion come after the war.

Matthews, Michael D. Head Strong: How Psychology is Revolutionizing War. Oxford University Press, 2013.

Dickstein, Benjamin D., et al. "Unit cohesion and PTSD symptom severity in Air Force medical personnel." Military Medicine 175.7 (2010): 482-486.