The saying I recall hearing is toughness in another term for training and training is another term for reducing the unknowns. The unknowns become the psychological outliers that generate trauma. There does seem to be a correlation between level of training and incidence of PTSD.

I'm particularly interested in at what point does "PTSD" change from being a normal reaction to sustained stress, and start being a genuine disease.
The DSM IV distinguishes between Acute Stress Disorder and Post Traumatic Stress Disorder, with the main difference being duration of symptoms. I think the thing to remember is that trauma is relative. For lack of a better analogy, stress is an emotional gas that will expand to fill the volume of its container. Training, experience, toughness, or whatever you want to call it will give you a larger “container”. Also, I can’t find the citation, but recall reading the estimate that 30 to 35% of soldiers experience acute or chronic PTSD. The rate among cops is somewhere north of 40%.