The mind can both heal you and hurt you!
Also the double-blind study is VERY important for ANY scientific study, because most scientists design a study because they predict a certain result, and that simply prediction might push them to interpret ambiguous data, or noisy data in the favor of their initial hypothesis.
You will notice that in all anti-gun studies there are NO Blinds in place. The researcher is paid to produce an outcome, and AMAZINGLY that outcome is the end result. Thankfully for those studies the methods are often so skewed you can see how only one outcome is possible!