Poor clinical education (or lack thereof) for DID and other dissociative disorders has been described in literature: "most clinicians have been taught (or assume) that DID is a rare disorder with a florid, dramatic presentation." Symptoms in patients are often not easily visible, which complicates diagnosis. DID has a high correlation with, and has been described as a form of, complex post-traumatic stress disorder. There is a significant overlap of symptoms between borderline personality disorder and DID, although symptoms are understood to originate from different underlying causes. Historical prevalence
Rates of diagnosed DID were increasing in the late 20th century, reaching a peak of diagnoses at approximately 40,000 cases by the end of the 20th century, up from less than 200 diagnoses before 1970. Initially DID along with the rest of the dissociative disorders were considered the rarest of psychological conditions, diagnosed in less than 100 by 1944, with only one further case reported in the next two decades. In the late 1970s and 80s, the number of diagnoses rose sharply. An estimate from the 1980s placed the incidence at 0.01%. Accompanying this rise was an increase in the number of alters, rising from only the primary and one alter personality in most cases, to an average of 13 in the mid-1980s (the increase in both number of cases and number of alters within each case are both factors in professional skepticism regarding the diagnosis).
The condition may be under-diagnosed due to skepticism and lack of awareness from mental health professionals, made difficult due to the lack of specific and reliable criteria for diagnosing DID as well as a lack of prevalence rates due to the failure to examine systematically selected and representative populations. Differential diagnoses
People with DID are diagnosed with five to seven comorbid disorders on average – much higher than other mental illnesses.Due to overlapping symptoms, the differential diagnosis includes schizophrenia, normal and rapid-cycling bipolar disorder, epilepsy, borderline personality disorder, and autism spectrum disorder. Delusions or auditory hallucinations can be mistaken for speech by other personalities. Persistence and consistency of identities and behavior, amnesia, measures of dissociation or hypnotizability and reports from family members or other associates indicating a history of such changes can help distinguish DID from other conditions. A diagnosis of DID takes precedence over any other dissociative disorders. Distinguishing DID from malingering is a concern when financial or legal gains are an issue, and factitious disorder may also be considered if the person has a history of help or attention-seeking. Individuals who state that their symptoms are due to external spirits or entities entering their bodies are generally diagnosed with dissociative disorder not otherwise specified rather than DID due to the lack of identities or personality states. Most individuals who enter an emergency department and are unaware of their names are generally in a psychotic state. Although auditory hallucinations are common in DID, complex visual hallucinations may also occur.