<P> Dissociation, the term that underlies the dissociative disorders including DID, lacks a precise, empirical, and generally agreed upon definition . A large number of diverse experiences have been termed dissociative, ranging from normal failures in attention to the breakdowns in memory processes characterized by the dissociative disorders . Thus it is unknown if there is a common root underlying all dissociative experiences, or if the range of mild to severe symptoms is a result of different etiologies and biological structures . Other terms used in the literature, including personality, personality state, identity, ego state and amnesia, also have no agreed upon definitions . Multiple competing models exist that incorporate some non-dissociative symptoms while excluding dissociative ones . The most widely used model of dissociation conceptualizes DID as at one extreme of a continuum of dissociation, with flow at the other end, though this model is being challenged . </P> <P> Some terms have been proposed regarding dissociation . Psychiatrist Paulette Gillig draws a distinction between an "ego state" (behaviors and experiences possessing permeable boundaries with other such states but united by a common sense of self) and the term "alters" (each of which may have a separate autobiographical memory, independent initiative and a sense of ownership over individual behavior) commonly used in discussions of DID . Ellert Nijenhuis and colleagues suggest a distinction between personalities responsible for day - to - day functioning (associated with blunted physiological responses and reduced emotional reactivity, referred to as the "apparently normal part of the personality" or ANP) and those emerging in survival situations (involving fight - or - flight responses, vivid traumatic memories and strong, painful emotions, the "emotional part of the personality" or EP). "Structural dissociation of the personality" is used by Otto van der Hart and colleagues to distinguish dissociation they attribute to traumatic or pathological causes, which in turn is divided into primary, secondary and tertiary dissociation . According to this hypothesis, primary dissociation involves one ANP and one EP, while secondary dissociation involves one ANP and at least two EPs and tertiary dissociation, which is unique to DID, is described as having at least two ANP and at least two EP . Others have suggested dissociation can be separated into two distinct forms, detachment and compartmentalization, the latter of which, involving a failure to control normally controllable processes or actions, is most evident in DID . Efforts to psychometrically distinguish between normal and pathological dissociation have been made, but they have not been universally accepted . </P> <P> According to the fifth Diagnostic and Statistical Manual of Mental Disorders (DSM - 5), DID symptoms include "the presence of two or more distinct personality states" accompanied by the inability to recall personal information, beyond what is expected through normal forgetfulness . Other DSM - 5 symptoms include a loss of identity as related to individual distinct personality states, and loss referring to time, sense of self and consciousness . In each individual, the clinical presentation varies and the level of functioning can change from severely impaired to adequate . The symptoms of dissociative amnesia are subsumed under the DID diagnosis but can be diagnosed separately . Individuals with DID may experience distress from both the symptoms of DID (intrusive thoughts or emotions) and the consequences of the accompanying symptoms (dissociation rendering them unable to remember specific information). The majority of patients with DID report childhood sexual or physical abuse, though the accuracy of these reports is controversial . Identities may be unaware of each other and compartmentalize knowledge and memories, resulting in chaotic personal lives . Individuals with DID may be reluctant to discuss symptoms due to associations with abuse, shame, and fear . DID patients may also frequently and intensely experience time disturbances . </P> <P> Around half of people with DID have fewer than 10 identities and most have fewer than 100; as many as 4,500 have been reported . The average number of identities has increased over the past few decades, from two or three to now an average of approximately 16 . However, it is unclear whether this is due to an actual increase in identities, or simply that the psychiatric community has become more accepting of a high number of compartmentalized memory components . The primary identity, which often has the patient's given name, tends to be "passive, dependent, guilty and depressed" with other personalities being more active, aggressive or hostile, and often containing a current time line that lacks childhood memory . Most identities are of ordinary people, though historical, fictional, mythical, celebrity and animal identities have been reported . </P>

What causes a person to have split personality