My own personal observation is that amongst people I know/have known, long-term mental illness is FAR more common amongst JWs than non-JWs. I'd be interested to know if other posters have made similar observations.
Depression definitely runs in my family amongst the JWs. My Dad is battling as is my cousin. My Mom battled for years -- she is gone now.
I noticed a lot of negativity, quite profoundly, once I moved out of my parents' home and started my own life as an adult. Every time we'd visit my folks my Mom would talk about how bad everything was. She had started chain smoking too. She smoked herself into COPD and died 11 years later. She once told me that her only pleasure in life was smoking. This was such a sad statement. She never went for any sort of help. It broke my heart to see her this way. I tried to get her to go for counseling but was unsuccessful.
I learned from this is that you can't help someone that doesn't want help. And what you focus on expands. When you concentrate your thoughts a certain way, these thoughts will manifest themselves in your life.
I once described my Mom as 'a disabled thinker' to my brother. At the time I wasn't sure of the source. I know now that it was the WTS / JWs.