Probing Question: How do schizophrenia and DID differ?

"What will I have for dinner? Is it going to rain later? I wonder what she meant by that."

Questions or comments like these silently passing through our minds reflect how most of us think; they’re normal. When the comments heard internally are the voices of other people, however, then psychiatrists suspect schizophrenia.

Among the myths surrounding schizophrenia, one of the most persistent is that it involves a "split personality," two separate and conflicting identities sharing one brain. A National Alliance on Mental Illness survey found that 64 percent of the public shares this misconception.

"It’s an extremely common misunderstanding," said Randon Welton, assistant professor of psychiatry at Penn State Milton S. Hershey Medical Center. "It’s based on the name. If you go to the Greek roots of the word -- 'schizein,' meaning 'splitting,' and 'phren,' meaning 'mind' -- you have 'split brain' or 'split mind.' However, the intended reference is to a split between rationality and emotions, not a split within a personality," said Welton.

Split personality, more properly, is an old name for multiple personality disorder, which is itself an outdated name for dissociative identity disorder (DID), an officially recognized but still controversial diagnosis. Welton said DID came to the public’s attention following the release of books and films such as "The Three Faces of Eve" and "Sybil," accounts of women who developed multiple, distinct personalities following severe abuse as children.

"I would describe DID as a trauma-based illness," Welton said. Those affected by it have "at least two and often more distinct identity states which each have fairly consistent patterns of relating to the environment." The American Psychiatric Association definition specifies that "at least two of these identities or personality states recurrently take control of the person's behavior."

By contrast, Welton describes schizophrenia as "a largely genetic illness, one that seems to be clustered within families. It seems to be more neurodevelopmental, influenced by how the brain develops. It usually presents in late teens to young adulthood and is more common than DID, with 2.2 million Americans living with the disease. Explained Welton, "You see a gradual, overall decrease in functioning with acute exacerbation, lasting weeks or months, of overtly psychotic symptoms -- unless they are caught and treated."

While trauma is associated with both disorders, Welton explained that "the traditional difference is that with schizophrenia, the trauma tends to follow the disease. It is a consequence of the illness; it is not causative. Trauma doesn’t make someone have schizophrenia, whereas for almost everyone with DID I’ve ever heard about, it is a reaction to the trauma." Schizophrenia is classified as a psychotic disorder and managed primarily through drugs, whereas DID is considered a developmental disorder that is more responsive to psychotherapy and behavioral modifications.

On the surface, the difference between the two disorders seems clear cut. But some psychiatrists, such as Brad Foote of the Albert Einstein College of Medicine, are warning their peers that it may be possible to confuse the two conditions early in the course of treatment. This may happen if voices of alternate personalities in a case of DID "leak through" and comment on events, or talk directly to the core, central personality, without completely taking it over.

"Traditionally, any time a patient reports hearing voices like this, it was a strong indication of schizophrenia," Welton said. "Psychosis is not a diagnostic key for DID but it is a common finding in that they will hear one personality talking to another or a personality commenting on them."

If these observations are accurate, Welton said, "it would be very easy to put that person into a psychotic disorder category because you did not ask the right questions or you didn’t ask in the right way."

Hearing voices may be more complicated than doctors or patients knew.