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IN 1791, a German doctor called Eberhardt Gmelin reported a bizarre case: one of his patients regularly transformed from a middle-class German woman into a French aristocrat. She would suddenly 鈥渆xchange鈥� her personality for the manners and ways of a French-born lady, speaking French perfectly but German as a Frenchwoman might. As her French self, she could remember everything she had said or done during her previous French 鈥渆pisodes鈥�. As a German woman, she knew nothing of her French personality.

Gmelin鈥檚 French aristo was followed by other similarly odd cases. Felida X, for example, had three different personalities, each with their own illnesses. One of them even had her own pregnancy, unknown, at first, to the others. These cases introduced the contentious multiple personality disorder (MPD).

For much of the 20th century MPD was eclipsed by Freud鈥檚 notions of hysteria and repression, but in the 1980s, for no obvious reason, it returned explosively. Between 1985 and 1995, an estimated 40,000 cases were diagnosed 鈥� twice as many as in the entire preceding century. But this time, MPD was considered more than just a psychiatric oddity. Under the label dissociative identity disorder (DID), doctors believed it was closely linked with childhood trauma.

The professionals were split. One camp argued that personality-switching was elaborate play-acting, encouraged by naive or needy therapists and fuelled by an emerging 鈥渧ictim鈥� culture. The other camp argued it indicated the surfacing of 鈥渁lters鈥� 鈥� selves created to carry the burden of traumatic events, which had then 鈥渟plit off鈥� and become buried. We now know the two theories are not necessarily in conflict: the brain is able both to create false memories and recover memories that seemed to be lost.

The International Society for the Study of Trauma and Dissociation was at the heart of it all because dissociation, the 鈥渟licing-up鈥� of experiences (and thus memories) into different streams of consciousness, was identified as the mechanism behind MPD. Once researchers were involved, dissociation became psychopathology and the notion of 鈥渘ormal鈥� dissociation more or less disappeared.

But the truth is we all dissociate to some extent: if we didn鈥檛, we would be overwhelmed by the barrage of stimuli that continually assaults our senses. In Multiplicity, I argue one effect of this normal dissociation is to create a normal multiplicity. So the gap between 鈥渃razy鈥� people with MPD/DID and the rest of us is a matter of degree.

With MPD/DID, the personalities are discrete, but those 鈥渕ultiples鈥� in the rest of us share perceptions, thoughts and emotions. Crucially, this means they also share many memories. While people with MPD have amnesic gaps for the periods when they assume another personality, the rest of us just have hazy patches in our recall and, tellingly, the odd moment when we look back on a deviation from our characteristic behaviour and wonder: 鈥淲hat got into me?鈥�

To understand how multiplicity comes about, we need to look at the dissociation spectrum. At one end is the everyday focusing of attention we call 鈥渃oncentration鈥�. Moving along there is the intense absorption in a subset of stimuli, or 鈥渇low鈥�. Then there is day-dreaming, mild trance-like states, and such oddities as out-of-body experiences and lucid dreaming. Further still is detachment, including depersonalisation (the feeling of being dislocated from one鈥檚 body or mind) and derealisation (feeling the world to be distant, or crushingly close, distorted or unreal).

Detachment is a defence mechanism, evolving from the 鈥減lay dead鈥� or 鈥渇reeze鈥� reaction to danger. Its adaptive function is obvious: in a car accident, an injured victim who can detach from their pain may be better able to escape. A doctor coming to help who detaches from their emotions may act more effectively. This normal response to an abnormal situation is only pathological if it continues beyond the threat or challenge.

Although we are not conscious of dissociated experiences at the time, they are nevertheless registered by the brain and may form memories. Later, such memories may be consciously recalled, resulting in a sort of 鈥渄elayed鈥� experience. In the 1970s, Ernest Hilgard, a psychologist at Stanford University, California, tested this hypnosis on patients who were, for various medical reasons, facing surgery without anaesthesia. Before their operations Hilgard put them into a trance and told them a 鈥渉idden observer鈥� would feel their pain for them. After surgery, the patients recalled having the operation but reported no pain. But when Hilgard hypnotised them again and asked the 鈥渉idden observer鈥� to speak, they reported the agony of the knife. The pain seemed to be stored in a 鈥渃ompartment鈥� the patient was unable to access ordinarily.

The compartmentalisation of MPD differs from other dissociative states in that entire, protracted experiences are stored separately, rather than just one component of an experience, such as pain. Prolonged experiences consist of sensations (the record of outside events) together with the emotions and thoughts we have at the time. Part of these internal events is a sort of background 鈥渉um鈥� of self-recognition (the knowledge of who you are, your past and so on) plus characteristic ways of seeing and responding to the world. These are the habits of mind and behaviour we recognise as a 鈥減ersonality鈥�.

Entire compartmentalised experiences will therefore hold a sense of identity and personality as well as sensations and emotions. So when one is recalled, the self that experienced it is recalled too. If it happened in childhood, the recaller will return to childhood, and while the recollection lasts will have no awareness of being adult or of subsequent experiences.

So personal identity and personality are functions of experience, not separate from it. In theory, we become a new personality in each situation. In practice, most experiences and the personalities they incorporate are similar enough to be partly integrated.

But as life becomes more heterogeneous, our personalities will be less closely conjoined. Compare the wide, fragmented experiences of children now with the narrow, continuous experiences of their grandparents. A child transported across cultures may still remain connected to her old life through her family. At school she speaks one language, with one set of opinions, habits and behaviours, while at home she behaves entirely differently. To avoid inner conflict, she flips between two personalities. The personalities are not 鈥渁cts鈥�: she feels and thinks totally differently in each case. Neither is faked, but neither is more 鈥渞eal鈥�. Increasingly, personalities will become compartmentalised as modern life drives us along the dissociative spectrum.

鈥淎s life becomes more heterogeneous, our personalities will be less closely conjoined鈥�

Those who associate dissociation with pathology may find this alarming, and there is evidence that more people are suffering from chronic detachment. But there is already evidence that normal dissociation can protect people from pain, infection and depression. People who report they are more 鈥渕ultiple鈥� suffer less from stress-related conditions. Say 鈥淛udy鈥� has a sporty personality, A, and an academic personality, B. If A loses a tennis match, A is annoyed, which results in tensed muscles and a backache. If A was the only personality Judy had she would be tense all day. But if she goes off to college, switching to Judy B, her muscles relax because B doesn鈥檛 care about the tennis match. So Judy suffers less than if she was only personality A.

It looks as if normal multiplicity could prove useful in helping people function in an increasingly complex world. But first we will need to recognise it as quite different from its pathological, 18th-century origins.

The Human Brain 鈥� With one hundred billion nerve cells, the complexity is mind-boggling. Learn more in our cutting edge special report.