Roger Ebert in his review of Benny and Joon described it as "a film that approaches its subjects so gingerly it almost seems afraid to them." (Ebert online) This is true especially when it comes to the main character Joon who though it is not said, it can be deduced that she is schizophrenic. I was clued into this after reading an article about schizophrenia where the symptoms for disorganized schizophrenia were child like behavior, showing barely an emotion, and problems thinking an expression thoughts clearly (PubMed online). These were all behavior that Joon exhibited throughout the movie.
If it wasn't for a scene where her brother Benny is visiting her shrink it would be hard to tell that Joon is mentally ill. Her behavior was child like and usually consisted of fits of anger which could easily be a tantrum. This theory was enforced further by the fact that her brother continued to treat her like a child. He hired babysitters, and often took her everywhere he went. His whole life was revolved around his sister. He turned down dates and halted budding relationships all because of Joon. Personally I thought his role was parental. He was very selfless by giving up his whole life to watch his sister. When the doctor in the movie suggested that he put Joon in a group home, Benny strongly protested even though it would have freed him from his babysitting responsibilities. Many people in his situation would have gladly put her in a group home, and visited her regularly. This can be compared to families with autistic children. Some families shun these kids because of their disabilities, shamed by the fact that they have an abnormal child, while other parents embrace these children. Those parent read up on their children's' condition and attend fundraising events to raise awareness and money for a cure. Benny's behavior was like the latter.
The psychological explanation in Benny and Joon was devoid. From the start of the movie, the audience was told that Joon had a mental condition, though not what it was. So of course most of the focus was on Joon, to me the movie had no point, and lacked a plot. It didn't seem to have any direction or flow and many movie reviewers agreed because many rated the movie three stars or less. The only interesting and original character in the movie was Sam, played by Johnny Depp. Sam gave the impressions of having a childlike mentality but every once in a while it would prove to the audience that he was an adult.