A study by the Rand Corporation concludes that contractors serving in dangerous posts in Iraq and Afghanistan had higher rates of post-traumatic stress disorder than soldiers. About half of the men paid to drive trucks in convoys through dangerous areas showed symptoms of PTSD. This is what I would expect. Soldiers need psychological support to survive in combat, and over the centuries armies have learned a lot about how to do this. The great mass of rituals built up in military life are largely intended to make soldiers feel that they are part of something much bigger than themselves; that they are supported by a whole nation, even when they are alone. When things go bad they have their comrades to rely on; if they do well they get medals, ceremonies, promotions. Armies put so much effort into this stuff because it helps soldiers.
In Iraq and Afghanistan we relied on private companies to supply many men in the most dangerous roles. I think this was a horrible idea for a long list of reasons. Now we can add another: deprived of the support that a well-run military provides to its soldiers, people are even more vulnerable to the stress of wartime.