The idea that multiple personalities can exist within the same body has long captured the Western imagination. From Three Faces of Eve to Sybil, from Pyscho to Raising Caine, from 60 Minutes to Oprah to One Life to Live, we are captivated by the fate of multiples who, divided against themselves, wreak havoc in the lives of others.

Why do we find multiple personality disorder (MPD) so fascinating? Perhaps because each of us is aware of a dividedness within ourselves: we often feel as if we are one person on the job, another with our families, another with our friends and lovers. We may fantasize that these inner discrepancies will someday break free, that within us lie other personalities—genius, lover, criminal—that will take us over and render us strangers to our very selves.

What happens when such a transformation literally occurs, when an alter personality surfaces and commits some heinous deed? What do we do when a Billy Milligan is arrested for a series of rapes and robberies, of which the original personality, Billy, is utterly oblivious? What happens when a Juanita Maxwell, taken over by her alter personality, Wanda, becomes enraged and commits a murder which would horrify Juanita? Who really committed these deeds? Are alter personalities people? Are they centers of consciousness which are akin to people? Mere parts of a deeply divided person? Who should held accountable for the crimes? Which is more appropriate—punishment or treatment?

In Jekyll on Trial, Elyn R. Saks carefully delineates how MPD forces us to re-examine our central concepts of personhood, responsibility, and punishment. Drawing on law, psychiatry, and philosophy, Saks explores the nature of alter personalities, and shows how different conceptualizations bear on criminal responsibility. A wide-ranging and deeply informed book, Jekyll on Trial is must reading for anyone interested in law, criminal justice, psychiatry, or human behavior.

ELYN R. SAKS is Orrin B. Evans Professor of Law, Psychology, and the Behavioral Sciences at the Law School of the University of Southern California.