I stumbled across an old paper from the 1980s by the neuropsychologist Humphreys and the philosopher Dennett on Multiple Personality Disorder. Dennett has published much in the philosophical literature on the self and his work is always thought-provoking but I have not til now seen papers by him on psychiatry. The paper does not draw firm conclusions on the controversy among psychiatrists and psychotherapists about aetiology but does consider the phenomena as they are described  by psychotherapists and suggests an interesting metaphor for the self.