Multiple Personalities/Dissociative Identity Disorder

I've seen many posts in this message board stating matter-of-factly that Sara has DID. Other than the contention that Sophia is a separate personality, where is the vidence for this assumption? We know Sara is an unreliable narrator who has experienced significant trauma in her life and is mentally ill as a result. However, mental illness does not in most cases equal DID. PTSD, schizophrenia, and a host of other disorders could account for the symptoms that are portrayed in the film. Sophia could have been a real person, about whom Sara's memories lay dormant until she returned to her childhood home, the source of her trauma. She could have been an imaginary friend, as some have proposed, who returned as a delusion/hallucination. Or, she coukd have been an outright delusion who appeared for the first time to help Sara consciously recall the abuse that took place, the memories of which she had previously repressed (supported by the fact that she claimed to have a bad memory and that many things from childhood were fuzzy - paraphrasing here). In fact, I contend this is a much more plausible explanation, given that, assuming DID is real for the sake of the film (it has actually been almost entirely discredited by the psychological community of late), two personalities would most likely not be interacting with each other or even aware of each other. The scene where Sara cuts Sophia (and is actually cutting herself) can be explained as a hallucination or psychotic break, etc.

I'm willing to entertain the possibility that it is in fact DID, but not simply because one person said it and others followed suit. I am happy to listen if someone can present a compelling argument.

Just because your father tried to eat you do we all have to be unhappy .... forever?