As I said in your first go at this, I don’t know the cause of my ptsd (combat PTSD, yes. Which aspect/event? No idea.), so I ticked the boxes of things I was exposed to.
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If this is an English as a Foreign Language paper... heads up, the word “reason” is one of our vague multiple meaning words.
Reason > Cause
Reason > Contributing Factors
Cause will ONLY be life threatening trauma or sexual violence / known as Criterion A trauma in the world of PTSD.
DSM5
Contributing factors will be far more varied (and include things like family / society support afterward, threats against your career, reintegrating into society... IE resilience stuff... great article on that here
Traumatic resilience: avoiding ptsd ) , etc.
Speaking of GREAT articles, that really provide an excellent overview on several different aspects... you may well have already read them, but just in case not? I cannot recommend these any more highly. Absolutely stellar job done by Anthony synthesising all of the available info (the manual on PTSD is over 700 pages long!) into concise and clearly laid out format. If you read nothing else, read the first 2 in this list.
Ptsd basics
The ptsd cup explanation
Understanding post traumatic stress disorder (ptsd)
Ptsd diagnosis
Ptsd therapies
Myths
Ptsd only affects military?
If trauma, then ptsd: busting the media fallacy of ptsd prevalence
Good luck with your paper! :D