Content Warning: This section discusses the effects of trauma and abuse as well as institutionalization and mental health treatment.

The Minds of Billy Milligan centers on the landmark case of William Stanley Milligan, the first person declared not guilty by reason of insanity because of dissociative identity disorder (DID). While the case undeniably set legal precedent and influenced legislation in multiple ways, Milligan’s story is equally important in the field of psychology given the characteristics of the mental illness he experienced.

Milligan’s case brought attention to and increased awareness of an illness that, at the time, was poorly understood. During Milligan’s childhood, “multiple personality disorder” was not even classified as a disorder in the Diagnostic and Statistical Manual (DSM), second edition; symptoms like what Milligan experienced were instead diagnosed as “hysterical neurosis, dissociative type” (Diagnostic and Statistical Manual of Mental Disorders, Second Edition). Nevertheless, Milligan was misdiagnosed as a teen; furthermore, mental health professionals ignored and invalidated his supposedly accurate diagnosis on multiple occasions as an adult. Milligan received the diagnosis of “multiple personality disorder” for the first time after his arrest, when the disorder was making its way into the third edition of the DSM. Despite this, at the Lima facility, Dr.