Why do 30% of American combat veterans suffer from Post-Traumatic Stress Disorder compared to only 4% of British combat veterans?
One suggestion is that Americans have internalized the idea that killing and being nearly killed causes psychosis much more than the Brits have. In fact, the argument is stronger than the linked article suggests, because the work of SLA Marshall is probably hokum. Which means that modern American PTSD rates contrasts not only to modern British rates but to the apparently lower rates in past American wars.
In recent years psychologists observed the emergence of a new psychosis, the amputee fantasy, that had never been observed before. Mental illness, it turns out, can be cultural.
Psychology is in the mind.