The multiplicity of selves becomes more intuitive as the time span increases. Social psychologists have found certain differences in how we think of ourselves versus how we think of other people—for instance, we tend to attribute our own bad behavior to unfortunate circumstances, and the bad behavior of others to their nature. But these biases diminish when we think of distant past selves or distant future selves; we see such selves the way we see other people. Although it might be hard to think about the person who will occupy your body tomorrow morning as someone other than you, it is not hard at all to think that way about the person who will occupy your body 20 years from now. This may be one reason why many young people are indifferent about saving for retirement; they feel as if they would be giving up their money to an elderly stranger.

One can see a version of clashing multiple selves in the mental illness known as dissociative-identity disorder, which used to be called multiple-personality disorder. This is familiar to everyone from the dramatic scenes in movies in which an actor is one person, and then he or she contorts or coughs or shakes the head, and—boom!—another person comes into existence. (My own favorite is Edward Norton in Primal Fear, although—spoiler alert—he turns out in the end to be faking.)

Dissociative-identity disorder is controversial. It used to be rarely diagnosed, then the number of reported cases spiked dramatically in the 1980s, particularly in North America. The spike has many possible explanations: the disorder was first included as a specific category in the 1980 version of the Diagnostic and Statistical Manual of Mental Disorders, just as an influential set of case studies of multiple personalities was published. And increased popular interest was fueled by the 1973 novel Sybil and its 1976 movie adaptation, which starred Sally Field as a woman with 16 different personalities.

Some psychologists believe that this spike was not the result of better diagnosis. Rather, they say it stemmed in part from therapists who inadvertently persuaded their patients to create these distinct selves, often through role-playing and hypnosis. Recent years have seen a backlash, and some people diagnosed with the disorder have sued their therapists. One woman got a settlement of more than $2 million after alleging that her psychotherapist had used suggestive memory “recovery” techniques to convince her that she had more than 120 personalities, including children, angels, and a duck.

Regardless of the cause of the spike, considerable evidence, including recent brain-imaging studies, suggests that some people really do shift from one self to another, and that the selves have different memories and personalities. In one study, women who had been diagnosed with dissociative-identity disorder and claimed to be capable of shifting at will from one self to another listened to recordings while in a PET scanner. When the recordings told of a woman’s own traumatic experience, the parts of the brain corresponding to autobiographic memory became active—but only when she had shifted to the self who had endured that traumatic experience. If she was in another self, different parts of the brain became active and showed a pattern of neural activity corresponding to hearing about the experience of a stranger.