1. Dissociative identity disorder (DID): The main character, Elizabeth (Phoebe Cates), struggles with dissociative identity disorder, which is characterized by the presence of two or more distinct personality states. In the movie, Elizabeth's alternate personality is a mischievous, child-like character named Fred (Rik Mayall), who appears to be a manifestation of her repressed childhood fantasies.
2. Childhood trauma: Elizabeth's DID is suggested to be a result of childhood trauma, specifically the death of her mother when she was young. The movie implies that the creation of Fred as an alternate personality was a coping mechanism for Elizabeth to deal with the emotional pain of her loss.
3. Depression and anxiety: Elizabeth displays symptoms of depression and anxiety throughout the movie. She feels isolated and alone, struggling to connect with others and feeling overwhelmed by her responsibilities. These symptoms are exacerbated by the presence of Fred, who often creates chaotic and unpredictable situations in her life.
4. Obsessive-compulsive disorder (OCD): Elizabeth's ex-boyfriend, Ricky (Tim Matheson), exhibits symptoms of obsessive-compulsive disorder. He is shown to have a strict daily routine and an obsession with cleanliness and order. His compulsions cause him significant distress and interfere with his relationships.
5. Narcissistic personality disorder: Fred, in his adult form, displays traits of narcissistic personality disorder. He is self-centered, arrogant, and has an inflated sense of self-importance. He lacks empathy for others and often manipulates and exploits them for his own amusement.
It's important to note that the portrayal of these psychological conditions in the movie is for entertainment purposes and may not reflect the accurate representation of these conditions in real life.