The short answer here is no. Multiple personalities (now called Dissociative Identity Disorder, or DID in the DSM) is classified as a type of dissociative disorder, while schizophrenia is a type of psychotic disorder. Very shorthand descriptions of these types of conditions are:
- dissociation involves a disconnection of some kind, in this case between parts of identity
- psychosis involves an addition of some kind – hallucinations, delusions etc.
From the perspective of the DSM they are entirely separate and distinct, with fundamentally different processes involved and treatments. There are certainly huge differences between many of the experiences.
Popular culture often mixes them up, which tends to enormously irritate people with either diagnosis. I have some degree of sympathy for the confusion however, because even the concept of what schizophrenia, or for that matter, multiple personalities, actually is changes quite regularly and I get that folks outside of psychiatry aren’t getting the memo and keeping up.
The longer answer is still no, with some qualifiers.
Schizophrenia roughly translates to split mind. This does not traditionally refer to the idea of split personalities, but instead to divided mental process or a split from reality. Schizophrenia is a fairly poorly defined cluster of symptoms that has changed significantly over the years and since the previous term ‘dementia praecox’. ‘Multiple personalities’ has also been understood in various different ways over the years – as an experience of spiritual possession, a subtype of schizophrenia where the person is in fact suffering from the delusion that they have other personalities, and so on.
Where things get really tricky, even with the current rigidly defined separation between these two conditions, is in the overlap of presentation or experience. And there are a lot of them. Firstly, Schneiderian First-Rank Symptoms, which were once thought to be extremely diagnostic of schizophrenia (and involve experiences such as thought insertion, thought withdrawal, and voices heard arguing) have been shown in some studies to be far more indicative of DID. What this means is that telling the two conditions apart on the basis of observing a person, or even learning what kinds of experiences they are having can be very difficult.
Secondly, psychosis and dissociation often seem to co-occur in my personal experience. Many people with a psychotic condition find that massive dissociation is part of the prodromal (or onset) phase, just prior to a major break. Some people with a dissociative condition, like myself, experience psychotic symptoms such as hallucinations. PTSD is an excellent example of this. Technically classified as an anxiety disorder, people diagnosed with it commonly experience both significant dissociative and psychotic symptoms.
Thirdly the whole area of voices, which I think is what really confuses things in popular culture. The DSM perspective is that voices are hallucinations, while alters are split off parts of personality. The fact that some people who have DID can hear their alters as voices blurs the two categories. Having some people experience their voices as stable personalities who perceive themselves as separate but alive, likewise. There is a considerable space here where people from both diagnostic categories meet. For more on this overlap, see Parts vs Voices. For a lovely description of working with voices as parts, see Creating a New Voice by Indigo Daya.
For some people, the diagnostic labels are very useful and important. It can be a great relief to have a name for distressing or confusing experiences, and I’m not in any way trying to take that away. These frameworks have their uses. But they also have limitations, and when you move beyond the boiled down Psych 101 spin, life is more complex than these discrete packages of symptoms can really capture.