Most studies of trauma and dissociation are cross-sectional rather than longitudinal, which means researchers can not attribute causation, and studies avoiding recall bias have failed to corroborate such a causal link. In addition, studies rarely control for the many disorders comorbid with DID, or family maladjustment (which is itself highly correlated with DID). The popular association of DID with childhood abuse is relatively recent, occurring only after the publication of Sybil in 1973. Most previous examples of "multiples" such as Chris Costner Sizemore, whose life was depicted in the book and film The Three Faces of Eve, disclosed no history of child abuse. Pathophysiology
Despite research on DID including structural and functional magnetic resonance imaging, positron emission tomography, single-photon emission computed tomography, event-related potential, and electroencephalography, no convergent neuroimaging findings have been identified regarding DID, making it difficult to hypothesize a biological basis for DID. In addition, many of the studies that do exist were performed from an explicitly trauma-based position, and did not consider the possibility of therapy as a cause of DID. There is no research to date regarding the neuroimaging and introduction of false memories in DID patients, though there is evidence of changes in visual parameters and support for amnesia between alters. DID patients also appear to show deficiencies in tests of conscious control of attention and memorization (which also showed signs of compartmentalization for implicit memory between alters but no such compartmentalization for verbal memory) and increased and persistent vigilance and startle responses to sound. DID patients may also demonstrate altered neuroanatomy.
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).