This was addressed in a study by Rona in the British Medical Journal, using U.K. data. They looked at screening for mental health problems prior to entrance into the military and whether that would predict future problems. If someone had a history of mental illness, depression, or anxiety, would that person have a higher likelihood of post-deployment mental health problems? Their conclusion was no. According to the study, this information didn't help predict who was going to experience problems.
This is the biggest challenge with mental illness in general. As with diabetes or heart disease, there are genetic risks. Childhood adversity, physical abuse, sexual abuse, family difficulties, exposure to alcoholism in childhood—these factors put people at higher risk when they have a traumatic event. At least that is what's thought. But at this point, the short answer is that we don't have a good understanding of who is going to develop mental health problems after severe trauma. This is the huge question. We tried to do a study on it, but we didn't have the necessary support.