Previously known as multiple personality disorder, dissociative identity disorder is a particularly infamous mental disorder characterized by a person who has at least two distinct and dissociated personalities or states of being. Dissociative identity disorder is especially hard to diagnose as it overlaps significantly with other mental illnesses including eating disorders, anxiety, PTSD, and substance abuse.

Over time, the average number of personalities exhibited by people diagnosed with DID has grown from two or three to 16. Individuals with DID experience memory loss while switching between personality states. In one case, as many as 4,500 personalities in a single patient were recorded.