The Information Cascade That Killed George Washington
For thousands of years, bloodletting was considered a valid form a medical treatment, and was prescribed for any number of illnesses, from fevers and headaches to pneumonia and epilepsy. Despite what must have been a complete lack of reliable evidence for the efficacy of this procedure, physicians throughout the world continued to use bloodletting regularly until the 19th century. As explained in the article linked below, the reasons for this widespread belief in bloodletting can be thought of as an information cascade on a very large scale.
Information cascades result when people trust decisions made by others more than they trust their own information and reasoning abilities. In the first century a Greek physician, Galen of Pergamum, wrote about the benefits of bloodletting as a method to resolve imbalances of the humours by removing that most dominant humour, blood. Due to the incredible amount of influence Galen held within the medical community as a result of his prolific writing, physicians throughout Greece and beyond began to consider bloodletting as a standard treatment for a number of diseases. In the first century, of course, most ordinary people did not have much knowledge of medicine, and even physicians were just beginning to understand the human body, so if Galen said something was true, no one was likely to question it. As the practice spread throughout Europe, the Middle East, and Asia, the cascade only grew. To help him make the decision about whether bloodletting was a valid treatment, each physician had the information recorded about his own patients, and whether the practice seemed to help or harm them, and the fact that the entirety of the established medical community was also practicing bloodletting. Even if one particular physician did not see much direct benefit from the treatment in his own patients, he would more likely trust the prevailing wisdom of the medical community, which continued to use bloodletting, than his own observations, and so continue the information cascade.
It was only when scientific studies on bloodletting proved it to do more harm than good that its popularity as a medical procedure began to wane. When doctors learned about the studies and trusted them over the long-standing belief in bloodletting, the information cascade stopped. The medical community had new information that was proven correct, and no longer had a reason to just rely on traditional but unproven ideas.
http://www.bcmj.org/premise/history-bloodletting