Many athletes credit drugs with improving their performance, but some of them may want to thank their brain instead. Mounting evidence suggests that the boost from human growth hormone (HGH), an increasingly popular doping drug, might be caused by the placebo effect. In a new double-blind trial funded by the World Anti-Doping Agency, in which neither researchers nor participants knew who was receiving HGH and who was taking a placebo, the researchers asked participants to guess whether or not they were on the real drug. Then they examined the results of the group who guessed that they were getting HGH when, in fact, they had received a placebo. That group improved at four fitness tests measuring strength, endurance, power and sprint capacity. The study participants who guessed correctly that they were taking a placebo did not improve, according to preliminary results presented at the Society for Endocrinology meeting in June 2011. “The finding really shows the power of the mind” said Ken Ho, an endocrinologist at the Garvan Institute in Sydney, Australia, who led the study. She maintains that many athletes are reaping the benefits of the placebo effect, without knowing whether what they are taking is beneficial or not.