Split personality disorder is the common name for a psychological phenomenon called dissociative identity disorder that often presents itself following trauma. This multiple personality disorder gained awareness in the 1970s with the publication of "Sybil" and the subsequent movie in 1976. Sybil Isabel Dorsett had up to 16 different personalities brought on by brutal abuse from her mother. If you have lost periods of time and people say you said something you don't remember, you could have symptoms of split personality disorder. While you can easily discern if you have symptoms, always see a doctor for a diagnosis and treatment.
Review recent events, taking note of any days or occasions you don't remember. For instance, it is Thursday and Monday was your birthday, but you can't remember what you did that day. Determine if you have blackouts, where your consciousness seems to jump from one place to another with gaps of time in between, or you find yourself at a location, but don't know how you got there. These are signs of having a split personality, which is much more extreme than simple memory loss or forgetfulness.
Ask your friends and family if you have been behaving radically different. Alternate personalities tend to take on their own appearance, habits, gestures, speech patterns, gender and sexual behavior.
Pay attention to people's reaction to you. Strangers may seem to know you or call you by a different name. They relay a series of events to you that you don't remember and inquire as to the change in your appearance or behavior.
Listen to the voice in your head. If it is your voice and yours alone, you may not have a split personality. If you hear multiple voices that all sound different and none of them are yours, you may be showing signs of dissociative identity disorder.
Look in your journal or other written documents that only you use. Check to see if the handwriting is your own or someone else's. If the handwriting changes, that could be a sign of an alternate personality.
Look in the mirror and see if you recognize yourself. This is another sign that you may have dissociative identity disorder.
Evaluate your general feeling of existence. If you feel cloudy or desensitized as if you are watching the world pass you by without becoming engaged in it, you may want to talk with a doctor or mental health professional to determine if you are experiencing split personalities.