Recently, this research came out, which finds that disturbances in the same gene play a role in 5 of mental illness (autism, schizophrenia, bipolar, major depression, and ADHD.) The study followed 60,000 people worldwide, half with diagnosis and half with none. You can read about it here:
http://www.nytimes.com/2013/03/01/he...ders.html?_r=0

Of course, this is just a first step and there will need to be follow-up studies. But I find this very promising and pointing us in the right direciton.