Yes, most of the studies have a lot of restrictions, to tease out the possible "confounding" effects. One I read yesterday started with 3,000 people who filled out a questionnaire and ended up with 32 people in the treatment group and 30 controls! Funny, the subjects were from SDSU and UCSD. This was nothing to do with yoga, though. I am doing the "hard science" part and my friend is doing the yoga part, as she is more of an expert in that.
It's hard to find a lot of people who don't take psychotropic or any other drugs, don't smoke, and haven't had counseling.