In the movie, there's a question of how much of the story we're seeing is actually real. There's even a moment in the movie where another character asks Arthur if the laughing episodes are real or if he's just faking it. It's a question the film pointedly avoids answering. Although, we do learn that, again, assuming that anything in the story is on the level, Arthur was the victim of abuse, and so it does appear he may be dealing with PBA as a result.