Not everyone who's had traumatic experiences has PTSD.

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The scientists have a theory, and it has to do with the root causes of PTSD, previously undocumented. As compared with the resilient Danish soldiers, all those who developed PTSD were much more likely to have suffered emotional problems and traumatic events prior to deployment. In fact, the onset of PTSD was not predicted by traumatic war experiences but rather by childhood experiences of violence, especially punishment severe enough to cause bruises, cuts, burns and broken bones. PTSD sufferers were also more likely to have witnessed family violence and to have experienced physical attacks, stalking or death threats by a spouse. They also more often had past experiences that they could not, or would not, talk about.

PTSS almost seems like a culture-bound syndrome of the modern West.

There are significant confounders here, as modern science-based psychology got started around the same time - and WWI really was very different from earlier conflicts, not least in its sheer scale. But the idea is nonetheless intriguing; the West really is quite different from traditional societies, along lines that could plausibly make folks more vulnerable to traumatic shock.