Chris Costner Sizemore has been called the most famous psychological case study in history. Growing up, her young mind was inhabited by a host of different personalities. Her extraordinary experiences gave rise to a newly defined mental illness, multiple personality disorder, and her story was turned into an Oscar-winning movie. Five decades after the first diagnosis are we any closer to understanding the mysterious rise of the alter ego? Chris Costner Sizemore talks to Stephen Sackur.