“Conventional medical wisdom has long held that placebo effects depend on patients’ belief they are getting pharmacologically active medication. A paper published in the journal Pain is the first to demonstrate that patients who knowingly took a placebo in conjunction with traditional treatment for lower back pain saw more improvement than those given traditional treatment alone.” https://www.sciencedaily.com/releases/2016/10/161014214919.htm
The placebo effect clearly can affect confidence in our understanding of psychology and medicine. The placebo effect is so strong that even when patients are told that they are receiving a placebo they show some signs of improvement.