Ptsd is more likely if you don't have as much social contact + can't express grief. Potentially 'cant express grief publicly'. If entire community subject to tragedy less rates of ptsd - even allievates ptsd symptoms in long term ptsd victims.

During war/after 911, suicide/depression/homicide rates drop. Potentially from being involved in a grand cause. Causes people to speak to each other. Our brains enjoy helping each other.

Much higher rates of mental health problems in human history. These increase with urbanization. Countries might focus too much on tearing themselves apart by every possible ethnic line etc.

Soldiers in ancient times got ptsd at lower rates, potentially due to; death/blood being more common, at camp they were usually safe whereas nowadays - never safe (drones, long range missiles, ied) so modern soldiers always always hypervigilant, killing was considered glorious, crying was allowed amongst men

concussions increase the odds of ptsd developing

35 days of uninterrupted combat, 98% have psychiatric disturbances.

Short term hypervigilance is probably advantageous - easily awaken, worried about event.