Dissociative identity disorder (DID) is a very rare dissociative disorder in which a person has at least two distinct personalities that control their behavior at different times. Previously known as multiple personality disorder (MPD), it often leaves patients with an inability to recall important recent and past personal information. It involves dissociation, a psychological defense mechanism in which anxiety-provoking thoughts, emotions or physical sensations are separated from the rest of the psyche.

An individual’s personality is made up of lasting patterns of perceiving, relating to and thinking about oneself and the surrounding environment. People with DID may present as though they have more than one personality state and may switch suddenly from one of these identities to another. These identities – known as alters – may vary tremendously. For example, one alter may be quiet and shy whereas another is loud and boisterous. In some cases, patients may have alters of different genders and various ages. Despite having multiple alters, it is important to remember that these various alternate states are all manifestations of one single person.

Patients with DID may believe they have up to 100 or more distinct personalities. About half of all patients experience 10 or fewer personalities. Each personality has its own personal history, self-image and identity (including a name). These personalities may be aware of one another, but just one personality controls a patient’s behavior at any given time.

Most patients develop DID during childhood, although the disorder is not always recognized during the childhood years. It is diagnosed three to nine times more frequently in women than in men, and women tend to have more identities (15 or more) than men (who average approximately eight identities). Many patients with DID report a history of childhood abuse (e.g., sexual abuse). Some patients do not have history of abuse, but have experienced a significant early loss (e.g., death of a parent), serious medical illness, or other very stressful events. Symptoms of DID may appear less frequently in patients who reach their late 40s and beyond, but may re-emerge during times of stress, trauma or substance abuse. Stress often triggers the transition from one personality to another.

In recent years, a rise in the number of DID cases has been reported in the United States. Some believe that a greater awareness of DID among medical professionals has contributed to a higher number of diagnoses of the disorder. Others believe the condition is becoming over diagnosed.