In my observation, the quantity and quality of training do favorably affect the number of PTSD cases by driving that number down.

However, I've noticed that it is impossible to predict what event will send even the strongest will around the bend. I've seen people blow off mind numbing events and seen a crying child send an old experienced soldier into la-la land. No way to tell what will be the proverbial straw...

That 30-40% figure also tracks with my observation over the years. I believe that figure is telling. If about that percentage have Sociopathic tendencies (and I suggest they do), they're unlikely to get combat related PTSD; stands to reason that an approximately equal percentage have the reverse of Sociopathy and are therefor going to be highly prone to develop PTSD.

Psychological selection for combat suitability should be viable -- if not right now, then soon...