Despite the dark jokes Marines crack about becoming “pink mist,” it seems obvious that repeated exposure to the horrors of war — seeing friends ripped apart by bombs or children inadvertently shot at checkpoints — would exact a psychological toll. Research has long shown that trauma is cumulative. When it comes to combat stress and suicide, though, the findings are mixed. “At this point in time, there does not appear to be any scientific correlation between the number of deployments and those that are at risk, but I’m just hard-pressed to believe that’s not the case,” Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, said during a conference in January. His hunch is supported by the Army’s battlefield survey of soldiers in Afghanistan and Iraq last year, which reported that the rate of mental-health problems such as depression, anxiety and PTSD rose with the number of combat tours.