It isn't known for sure what causes dissociative identity disorder (that's the technical term for it), but it's thought that some combination of the following factors is responsible:

1. Extreme stress
2. Past mental traumas
3. Insufficient childhood nurturing/child abuse (a very significant percentage of people with the condition report such, and it's been confirmed objectively)
4. Some natural ability to mentally separate memories from consciousness.
Source(s):
http://en.wikipedia.org/wiki/Split_personality#Causes
https://scholarsbank.uoregon.edu/dspace/bitstream/1794/1837/1/Diss_10_1_9_O...
http://books.google.com/books?id=3SQrtpnHb9MC&pg=PA527&lpg=PA535&am...
http://www.psyter.org/allegati/180/Kluft.pdf
http://www.merck.com/mmhe/sec07/ch106/ch106d.html

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Split personality, like a Dr. Jekyll and Mr. Hyde split personality remains a mystery. There has been no scientific evidence to what causes it in each individual case, so they cannot identify the cause exactly. However, such things as overwhelming stress, traumatic antecedents, insufficient childhood nurturing, and an innate ability to dissociate memories or experiences from consciousness are all possible causes.

It's a disease in the mind, or more so of a disorder where the root cause is not traceable.

Definition of Split Personality: “A relatively rare dissociative disorder in which the usual integrity of the personality breaks down and two or more independent personalities emerge”.

Explanation: There is no category or phenomenon in psychiatry called split personality. The term is commonly used in popular language to indicate a contradictory or drastically and dramatically alternating type of behavior of the"Jekyll and Hyde" type. It is often confused with the medical illness of schizophrenia because the etymology of the latter (from the Greek schizein, to split + phren, mind) suggests, misleadingly, that schizophrenia is a type of split personality. In schizophrenia, however, the splitting is within one single personality as the individual's thoughts, feelings and emotions are seriously and confusingly disconnected from each other in a chaotic and random fashion. Schizophrenic individuals, far from having split or multiple personalities, actually have a great struggle maintaining the coherence and integrity of even a single self.
Source(s):
http://sawaal.ibibo.com/self-development/what-split-personality-464254.html
http://ezinearticles.com/?Split-Personality---A-Myth-or-a-Reality?&id=4...
http://en.wikipedia.org/wiki/Split_personality

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As with most mental disorders, research is still out on most causes. Disease, chemical imbalance, defense mechanism, result of trauma, etc.

Splitting off one or more distinct personalities used to be called Multiple Personality Disorder (in DSM-III), but is now called Dissociative Idenity Disorder (in DSM-IV-TR).

In DSM-IV-TR, to qualify for a DID diagnosis:

A. The presence of two or more distinct identities or personality states.

B. At least two of these identities or personality states recurrently take control of the person's behavior

C. Inability to recall important personal information that is too extensive to be explained by ordinary forgetfulness.

D. The disturbance is not due to direct physicological effects of a substance or general medical condition.
Source(s):
http://en.wikipedia.org/wiki/Multiple_personality_disorder

DSM-IV-TR (300.14 Dissociative Identity Disorder)

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Split Personality or multiple personality disorder is a mental condition/disease. This is the laymans definition of the condition according to Wikipedia the peoples encyclopedia. Dissociative Identity Disorder (DID), as defined by the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM), is a psychiatric diagnosis that describes a condition in which a single person displays multiple distinct identities or personalities (known as alter egos or alters), each with its own pattern of perceiving and interacting with the environment. The diagnosis requires that at least two personalities routinely take control of the individual's behavior with an associated memory loss that goes beyond normal forgetfulness; in addition, symptoms cannot be due to substance abuse or medical condition".

Split personality usually constitutes of two distinct conditions Schizophrenia and Dissociative Identity Disorder (DID). The above definition provides the general definition of the later condition Dissociative Identity Disorder (DID).

Schizophrenia on the other hand is defined thus by the website schizophrenia dot com."Schizophrenia is a chronic, severe, and disabling brain disease. Approximately 1 percent of the population develops schizophrenia during their lifetime � more than 2 million Americans suffer from the illness in a given year. Although schizophrenia affects men and women with equal frequency, the disorder often appears earlier in men, usually in the late teens or early twenties, than in women, who are generally affected in the twenties to early thirties. People with schizophrenia often suffer terrifying symptoms such as hearing internal voices not heard by others, or believing that other people are reading their minds, controlling their thoughts, or plotting to harm them. These symptoms may leave them fearful and withdrawn. Their speech and behavior can be so disorganized that they may be incomprehensible or frightening to others".

While Wikipedia has this definition for the same condition."Schizophrenia is a psychiatric diagnosis that describes a mental disorder characterized by abnormalities in the perception or expression of reality. It most commonly manifests as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking with significant social or occupational dysfunction. Onset of symptoms typically occurs in young adulthood,[1] with approximately 0.4–0.6% of the population affected".

So basically yes Split personality or multiple split personality is a medical condition/disease that has very many causative factors that you can find discussed in the links provided below.

These links provide but a general informative, overview of the conditions that constitute Split personality .If you think you, or someone you know might be suffering from such a a condition then it is strongly advised that you seek a Professional opinion from a qualified medical practioner/Psychiatrist in your area. I hope this information is helpful to you.
Source(s):
http://en.wikipedia.org/wiki/Dissociative_identity_disorder http://www.schizophrenia.com/family/sz.overview.htm http://en.wikipedia.org/wiki/Schizophrenia

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