Many years ago I read a book by Dr. Larry Dossey, called: Healing Words: The Power of Prayer and the Practice of Medicine. If you have not read it, you may enjoy it. Dr. Dossey relates many instances of the placebo effect that have gone through modern scientific studies, and the results may surprise many.
All ancient cultures used amulets, icons, potions, chants, feathers and the like which were to heal the people, and often did so, along with herbs and other remedies they carried in their tool-kits.
Here is a new study about the efficacy of placebos that I hope you will enjoy.