IN THE SHOWTIME series United States of Tara, actress Toni Collette plays Tara Gregson, a Kansas mother who has dissociative identity disorder (DID), known formerly as multiple personality disorder. As with others with DID, Tara vacillates unpredictably between various personalities, often referred to as alters, over which she does not have control. One of these alters is a flirtatious and flamboyant teenager, another is a traditional 1950s housewife, and a third is a boisterous Vietnam War veteran.

Many films, such as The Three Faces of Eve (1957) and Me, Myself, and Irene (2000), similarly portray individuals as possessing more than one personality. Some of them even confuse DID with schizophrenia [see “Living with Schizophrenia,” by Scott O. Lilienfeld and Hal Arkowitz; Scientific American Mind, March/April 2010]. Even the revised fourth edition of the American Psychiatric Association’s diagnostic manual, published in 2000, specifies the core features of the disorder as the “presence of two or more distinct identities or personality states.” Yet despite the pervasive popular—and professional—portrayal of this disorder, research casts doubt on the idea that anyone truly harbors more than one personality.

Dramatic Differences

Plenty of evidence supports the idea that DID is not merely a matter of faking and that most people with the condition are convinced that they possess one or more alters. Although a few DID patients have only one alter—the so-called split personality—most report having several. In a survey conducted in 1989 by psychiatrist Colin Ross, then at Charter Hospital of Dallas, and his colleagues, the average number of alters was 16. Interestingly, that is the same number of alters purportedly possessed by Shirley Ardell Mason, the woman known as Sybil in the 1973 best-selling book and two made-for-television movies that popularized the diagnosis of multiple personality disorder. (Later evidence emerged suggesting that Sybil’s primary therapist encouraged her to display multiple personalities, creating a huge sensation.) In rare cases, the number of alters may reach into the hundreds or even thousands.

Differences among alters can be nothing short of astonishing. Alters within the same patient may be of different ages, genders, races and even species, including lobsters, ducks and gorillas. There have even been reported alters of unicorns, Mr. Spock of Star Trek, God, the bride of Satan, and Madonna. Moreover, some practitioners claim that alters can be identified by objective characteristics, including distinct handwriting, voice patterns, eyeglass prescriptions and allergies. Proponents of the idea of multiple personalities have also performed controlled studies of biological differences among alters, revealing that they may differ in respiration rate, brain-wave patterns and skin conductance, the last being an accepted measure of arousal.

The question of whether people can harbor more than one character has important legal and therapeutic implications. If they can, and if patients are often unaware of their alters’ actions, a legal defense of “not guilty by virtue of DID” may be justifiable. Other scholars have argued that each alter is entitled to separate legal representation. As professor of law Ralph Slovenko of Wayne State University noted in 1999 in an article, some judges have even required each alter to be sworn in separately prior to testifying.

In treating these patients, many therapists try to get them to integrate their discrete personalities into a coherent whole. In doing so, they may help patients contact “undiscovered” alters and forge lines of communication among alters. For example, Ross has advocated naming alters and holding “inner board meetings” in which they can converse, share opinions and provide information about missing alters. Psychiatrist Frank Putnam of Cincinnati Children’s Hospital has argued for the use of DID “bulletin boards” on which alters can post messages for one another in notebooks or other convenient venues.

Putting the Pieces Together

Despite such practices, persuasive evidence for discrete coexisting personalities in individuals is lacking. The reported distinctions among alters are mostly anecdotal, unconfirmed and difficult to interpret. For instance, the handwriting and voices of people without DID may also vary over brief periods, especially after a mood change. And disparities in physiological reactions, such as brain waves or skin conductance, could be similarly attributable to differences in mood or thoughts over time, according to University of Arizona psychologists John J. B. Allen and Hallam L. Movius. Individuals with DID almost surely experience dramatic psychological changes across situations, so it would be surprising if their physiology did not change as well.

If alters are truly distinct personalities, they should have memories that are inaccessible to other alters. Yet Allen and psychologist William G. Iacono of the University of Minnesota reported in a 2001 review that although most direct memory tests—such as asking patients to recall a list of words in one alter state that they had previously encountered in a different alter state—reveal a lack of transfer of memories across alters, subtler tests usually reveal that memories formed by one alter are in fact accessible to others. In these less direct tests, which tend to be more sensitive and less prone to intentional manipulation of responses, subjects may be asked, for example, to complete a word such as “kin_” after an alter was presented with a related word, say, “queen.” Most subsequent studies bear out this conclusion, suggesting that alters are not distinct entities.

If alters are not discrete personalities, what are they? One hint: individuals who develop DID often meet the diagnostic criteria for borderline personality disorder, bipolar disorder and other conditions marked by instability. Indeed, a review in 1999 by one of us (Lilienfeld) and his colleagues found that between 35 and 71 percent of patients with DID also have borderline personality disorder. Understandably, then, many individuals prone to DID are bewildered by their unstable moods, self-destructive behavior, impulsivity and erratic relationships and are seeking an explanation for these disturbances. If psychotherapists or others ask suggestive questions such as “Is it possible that a part of you you’re not aware of is making you do and feel these things?” patients may become convinced that their mind houses multiple identities.

Data show that many therapists who treat DID patients use hypnosis, which may fuel these people’s difficulties in distinguishing fantasy from reality. Thus, DID may reflect an effort by individuals to make sense of extremely puzzling behaviors and feelings, a hypothesis proffered by the late psychologist Nicholas Spanos of Carleton University.

If so, techniques for making alters talk to one another may backfire, encouraging patients to falsely believe that the varied thoughts and feelings reside separately in their minds, often rendering them more difficult to integrate. For example, a patient could become convinced that one of her alters is responsible for her intense anger toward her husband, causing her to disregard her true feelings.

A better approach would be to help patients understand that their painful psychological experiences are created not by different personalities but by different aspects of one troubled personality. That way those suffering could begin to come to grips with these experiences and recognize that their thoughts and feelings are genuinely their own.