Jundo's blog recently linked this PDF file debunking Ken Wilber. When I read it, I noticed there was a section (in Chapter IV) on the negative side effects of meditation.
Long-term meditators reported the following percentages of adverse effects: antisocial behavior, 13.5%; anxiety, 9.0%; confusion, 7.2%; depression, 8.1%; emotional stability, 4.5%; frustration, 9.0%; physical and mental tension, 8.1%; pro-crastination, 7.2%; restlessness, 9.0%; suspiciousness, 6.3%; tolerance of others, 4.5%; and withdrawal, 7.2%....
I'm not quite sure where this data came from? I think it was a few late 70s/ early 80s TM studies? Maybe? (I had a bit of trouble understanding this eBook.)

Does anyone know of any recent information on adverse effects of zazen? Because I'm a little confused now. :?