When I was in the hospital, one of the education classes I attended was taught by a really awesome and knowledgeable psychologist. According to him, research has shown that for those of us with mental illness, at least one other relative will be found with mental illness; however, that can be as far back as 7 generations statistically which makes it hard to discover since often older generations didn't seek out professional evaluation/help for mental illness. For chemical dependency/addictions, statistically you rarely have to go back more than 3 generations to find a relative with addiction. The one thing I never got from him was the actual studies/literature he was getting that information from, but it does seem to illustrate the difficulty of discovering the family correlation for serious mental illness; it just may have been so far back and most of us know very little of our families beyond 3 or 4 generations at best.