In both The Exorcist and Head Full of Ghosts, the possessed characters start their affliction with some sort of mental unwellness. Regan, in The Exorcist, sees doctor after doctor, trying to find some “logical” explanation for her actions and behaviors. In Ghosts, Marjorie is introduced as going to a psychiatrist near the beginning of the book. A painfully obvious common thread, but it’s there.
In a separate class, I was given a reading titled Monster Culture (Seven Thesis) by Jeffery Jerome Cohen, a piece which goes into detail of seven commonalities the author observed in various works with monsters in them. Thesis 4 is titled The Monster Dwells at the Gate of Difference, basically, the monster exemplifies what is different and scary in the world, and makes it manifest. While maybe not intentional, I do believe that for both The Exorcist and Head Full of Ghosts, this ‘difference’ is mental illness.
While mental health has come a long way, in even just around 50 years (from 1970 to the present day), there is still a stigma of fear and wariness around those who have mental health issues. In The Exorcist we see doctors clamoring to explain away Regan’s illness (I’ve never seen a movie use the word temporal lobe so many times in such a short period), as if to claim that there was no possible way that Regan’s brain could make her behave like that regularly. In Ghosts, Marjorie’s father is convinced she just needs God to solve her problems. While yes, a good priest can make an excellent counselor, the idea that a counselor is not working after a few visits ignores the reality of mental health treatment, which can be a slow and arduous process of therapy, testing various dosages of medicines, and more therapy. The spectre of an unhealthy brain is still scary to many people, something that both of the works show.
Also, I feel it necessary to mention the what I found while Googling the real exorcism The Exorcist was based on. This is the Wikipedia (yes I know it’s not a great source but it sums it up well) page on the exorcism of Roland Doe, the exorcism the book and subsequent movie was based on. In it, we find that the consensus about the boy was not that he was taken over by some demon, but that he was merely a bratty small child who threw tantrums every time he was not the center of attention. It makes you think, what else have we tried to explain away with ‘demons’?