I just started reading a new book I got the other day, "risk factors for posttraumatic stress disorder". The book is written by a collective group of world trauma experts, or so called, and is based on the neurological and genetic predispositions for determining PTSD. The book contains lots of studies performed, and then some new ones by this group. Upon immediately getting into the introduction I new every study was biased before they began. Nice to know stuff, excellent reading if your that way inclined, though the studies themselves are biased to find predisposition. They used people who had endured trauma in their life and studied their past history, families, etc. They went looking for predisposition factors, and they found what they went looking for.
Now if they went looking from the same group factors to disprove predisposition, they would have also found those. Except what they state towards those people, are they didn't meet the study criteria and where removed. Those who did not show what they wanted in trauma got removed. Easy to do that.... taint your group by looking for exact specifics, the specifics you need to find the answer you want, and you will achieve your end result. Funny... they found their end result, being predisposition factors and genetic traits. They removed anyone that didn't show such factors though as they has traumatic pasts also, but didn't meet their subject criteria.
So here are a bunch of supposed world experts proving something that they tainted the result right from start, yet the intent was to write a book and publish some papers. They achieved that... but everything else was pretty useless other than to develop a drug or a technique maybe which would also be useless.
How to Disprove This Theory
If you got 20 subjects of which the only criteria was that they had not endured what is classified as "extreme traumatic events" within life as defined to meed for something like PTSD. You study their lives, they grew up quite normal, got a kick in the arse when bad, went through school, all the typical things we do and grow with that makes us who we are. Nothing excessive or prolonged, nothing extreme, just typical life growing up.
Take those 20 people and using previous studies for predisposition or genetics you could ascertain maybe a couple might be higher to get PTSD than any others. You find out from each what they would find traumatic in their lives, extreme that is. A women may be fine being shot at or within a war zone, yet if you took her child and killed them her brain would break, PTSD would develop. A male the same thing. Even rape, expose them to it, torture, a multitude of traumatic events.
What you would find at the end is that all 20 would obtain PTSD if you pressed them hard enough with the right traumatic event to them as individuals. A male who might be all tough and strong, put him in war and he breaks. Even worse, rape him and he develops PTSD now. A woman may be raped and not find that traumatic enough to develop PTSD, yet put her in war zone and that is traumatic enough for her. Again, bring children into it... you could see the traumatic range could be quite extreme and that it is based on an individual level to what is traumatic enough for one person uniquely.
You could get all 20 of those subjects to develop PTSD by applying the right extreme traumatic events within their lives. Again, you could test all 20 and only two had some genetic predisposition to PTSD, though you could actually get all 20 to get it. You could even take the two who showed under other studies a predisposition, and instead expose them to every traumatic event that they did not perceive traumatic to themselves, and those two could walk away the only one's without PTSD.
You can figure for yourself just how easy it is to taint a test. Not all studies show bias, the majority do though.