The findings appear online in the journal Psychological Trauma: Theory, Research, Practice, and Policy.
The study authors are with the Behavioral Science Division of the National Center for PTSD, based at the VA Boston Healthcare System, and with Boston University School of Medicine.
They identified three distinct phases of the Iraq War, based on previous reports. Then they analyzed whether veterans who fought during the insurgency phase, during which more guerilla-style tactics were used, were more likely to develop PTSD than those who deployed during the initial invasion phase of the war, or the more recent surge phase.
The study found that among the men - about half the overall group - the insurgency-phase veterans were more than twice as likely to have a diagnosis of PTSD, compared with those who served in either of the other two phases.
The finding held true even after the researchers adjusted for a range of other demographic and deployment-related risk factors.