Psychological or mental health topics are always interesting to research, but be sure that your instructor accepts this kind of topic. There generally needs to be some disagreement about the cause to make this paper work. If the cause is considered clear and uncontroversial, then you would be writing an Explaining essay rather than a cause argument essay. Here are some other possibilities:
1. What causes schizophrenia?
2. What effect does having a relative with schizophrenia have on families?
3. What causes obsessive-compulsive disorder? How does having this disorder affect a person's life and work?
4. What effect does this OCD have on a person's family relationships and friendships?
I have more topics on mental health here: http://hubpages.com/humanities/Easy-Essay-Topics-i...