We've found it very helpful to think of a brain as "containing zero or more consciousnesses" in general. Brains sort of consist of a mass of conscious material and it was never actually decided by evolution that there should only be one conciousness in there. (Though, I think having zero is not something that should normally happen... poor beans...)
Mechanisms supposedly unique to systems, like switching, aren't actually truly unique to systems, and just so happen to be something that both plural brains and even some singlet brains will develop completely independently of each other. I know at least one person who will have sudden mental shifts into the personalities/mindsets of their characters. They're decidedly non-plural (believe me, I have spoken to them about this for hours on end), but it could potentially be argued that this is a form of non-possessive switching in a singlet... between identities that are not dissociative in nature.
It really makes one think about whether brains were really only intended to contain one consciousness, if so many of them are able to develop switching and other plural mechanisms all on their own that are so similar across different systems. Sometimes without even knowing about their own plurality, let alone the phenomenon called plurality.
I still remember what it was like when we first discovered that "plurality" was A Thing That Other People Have. It's difficult to describe it concisely. We had previously called it "multi-personality" for around four years, but suddenly we had the labels to describe and understand it better, and also a bunch of validation. Before we had learned about plurality, we had never met (or noticed) any other plural systems in the wild. That changed quickly.
We're partial believers in the the theory of structural dissociation of course, but we feel that a lot of literature in general excludes the possibility of plurality outside of a dissociative disorder, even though plurality can happen on its own all the time, with or even without trauma (for example, catharigenic or parogenic plurality).
A slightly warmer take is that it may be possible for a brain to develop the ability to form structural dissociation later in life even if it was not forced to manifest during childhood. There is evidence that systems with dissociative disorders have the ability to split further later in life, generating additional structural dissociation because their brain knows how to create it.
There is no evidence proving that singlets can never develop this ability; only the current theory of structural dissociation, which implies that childhood trauma and integration failure is the main (or perhaps only) mechanism through which one may end up with structural dissociation.
This is not, however, the same as saying that one can develop the same DID as what is generally being studied right now. It is still dangerous to treat parogenic or even some/most catharigenic systems as sources of truth about DID, because the root cause can be much different, even if the symptoms manifest similarly (and even if they actually meet the criteria for a DID diagnosis, including distress).
We satisfy the diagnostic criteria for DID as specified in the DSM-5, but believe we are mostly catharigenic with only around one actually-traumagenic member (caused by emotional distress much later than childhood). Parogenic members have never been able to persist long for us, but some have existed for brief periods, it's just that we haven't managed to create one with a strong enough reason to exist.
That makes a lot of sense to me. I don't consider myself plural, but i have had some plural-adjacent experiences and i feel like i could under some circumstances become plural.
