I'll add a few things I know from therapy and reading. Just a heads up, I talk only about traumagenic dissociative disorders because that's what I know, that doesn't mean I don't acknowledge the existence of other forms of plurality and system origins, I'm just not able to talk about it.
According to my psychiatrist, who is not specialized in DID but in trauma disorders, DID/OSDD/pDID and other forms of traumagenic dissociative disorders are present in between 1 to 2% of the population.
It doesn't need to stem from "extreme" trauma but from repeated trauma mostly, no matter the severity of it. Things as "mundane" as loneliness or emotional neglect can cause it because it is most and foremost a coping mechanism. If you are more enclined to dissociate to cope, you are more enclined to develop DID even from "mild" experiences. As shown in the book "The Body Keeps The Score" about PTSD, two people can experience the exact same traumatic event and one is going to dissociate to cope, while the other will not.
That's why some people will go through extreme trauma and not develop any dissociative disorders, while some others will experience milder experiences and develop DID/OSDD. It's kind of a matter of how your brain responds to things more than the events itselves.
Still, according to my psychiatrist, the issue with DID/OSDD, etc, is that it is WILDLY underdiagnosed. People love to say that "everybody has DID," but in fact, psychiatrists are absolutely not trained to diagnose these disorders. My therapist told me that she had to educate herself and that she wouldn't have even thought of doing it if she wasn't specialized in trauma disorders. They don't have any classes about it in school, or barely so, and most of them don't have any tools to diagnose nor treat these disorders. So, the people who are the most likely to actually get diagnosed are the most severe cases who are going inpatient or are hospitalised because they can't function as they are.
That doesn't mean that a system that is functional is less valid than another that has to be in and out of treatment. It just means, like any other illness/disease, that severity is on a scale, dissociative disorders and trauma disorders are a spectrum, and it fluctuates.
We knew we were a system years even before we stumbled upon the online DID/OSDD community. Some other systems are not aware they are one until they get a diagnosis. It just depends. At the end of the day, all you really have to think about is being able to live your everyday life without it being a constant struggle. That's what matters. But saying that DID is an "extremely rare disorder" caused only by "absolutely horrific extreme unfathomable traumas" is false and contributes to the underdiagnosis and, therefore, lack of care that people with these disorders face.
