I do remember encountering a guy once out in field service who was what they would call me today: an apostate. Or, maybe he wasn't ex-JW, but knew a lot about them.
Though unverifiable, he maintained that there were more JWs as a percentage in mental institutions than any other religion. That threw me. To my trained JW mind, I hypothesised that it might be because we preach to them and convert them. Anyone else heard of this phenomenon?
Though thinking about it later, there were many people I knew in the Kingdom Hall who were not exactly "all there". When you think about it, there are a lot of imbalanced people out there who probably don't get attention and have few friends. Maybe they're depressed or lonely. When this group comes knocking on your door, offering to have you come join these friendly people, it's attractive.
My father once had a bible study with a guy who was nice enough, but a bit strange. Never really progressed too far. My dad didn't seem to care. He had his study he could count on his time sheet and was proud of that and kept the study going. This guy, however, probably appreciated the company more than anything. He was a wierd, dorky guy, probably in his late 40's who lived at home with his mother before she died. He probably had very few friends and welcomed the company.
I don't know that the organization necessarily causes all of the mental illness, though I'm certain there are enough factors to cause enough breakdowns (strictness, mind having to continually accept new light, etc.). I do think, however, that the organization, by it's active recruitment, becomes a magnet to people who are already mentally ill and perhaps haven't got the prescense of mind to see what they're getting themselves into.
-j.w.s.