Content Warning: This section mentions sexual assault, death by suicide, trauma and abuse, and institutionalization and mental health treatment.

The book’s central figure, William Stanley “Billy” Milligan (1955-2014) was the first person in the US declared not guilty by reason of insanity because of dissociative identity disorder (DID), or “multiple personality disorder,” as it was termed at the time. Milligan was the second child of Dorothy Turner and Johnny Morrison. He had an older brother, Jim, and a younger sister, Kathy. Johnny, a musician and comedian, was plagued by mental health struggles himself and died by suicide when Milligan was young; Dorothy remarried twice, and Milligan’s second stepfather, Chalmer Milligan, was instrumental in triggering Milligan’s illness. Milligan asserted that from the age of nine he was routinely abused, sadistically and sexually, by Chalmer. Based on the Teacher’s account, although Milligan displayed some dissociation earlier than this, Chalmer’s abuse coincided with the first major dissociation Milligan experienced, leading to the development of 24 different alters. This is consistent with the understanding of “multiple personality disorder” in Milligan’s time. Mental health professionals who examined Milligan then posited that the abuse was the possible cause of the illness.