Why was blood letting used for so many ailments for so long, despite being ineffective?

by TheNonDuality
wotan_weevil

Today we know that bloodletting has no useful medical function, apart from treating a small number of conditions (e.g., hemochromatosis). However, there is a long history across many continents of bloodletting being thought to be effective. Bloodletting was practiced across time from ancient Egypt and Mesopotamia to the 19th century by doctors as a medically-accepted technique, and is still used today:

It has been used from Western Europe through the Middle East and India to China. It has been used in Africa. It has been used in the Americas, both pre-Columbian and post-Columbian.

It has seen such widespread use because both patients and doctors believed that it was effective. If it doesn't work, why did people believe it worked? Why do some people still believe it works (see video linked above)? There are three major reasons.

First, people are bad at doing statistics in their head. People believe it works because they have seen cases where patients got better after bloodletting. Even if something has no effect at all, people will see patterns where none exist, and will infer a causal connection. This is not limited to bloodletting or even medicine, and has contributed to the belief in the effectiveness of a wide range of magical practices and rituals.

Further reading on this:

Second, even if a treatment has no effect, there are some confounding factors that can make it appear to work. Two key factors are: (a) patients who survive are more likely to tell their story than patients who die, and (b) patients will often see a doctor at the peak of their symptoms. While they would naturally have started to get better, they will credit the treatment with their recovery.

To avoid the above two problems, it is important to compare the response of an experimental group to the treatment and a control group, and use proper statistical analysis on the results.

Third, the placebo effect works. If patients believe that they have been treated, they will tend to respond positively. "Dramatic" treatment tends to produce a stronger placebo effect, e.g., if patients are given a placebo pill and told it costs $10 per pill, they will show a stronger response than to a 5 cent placebo pill. Bloodletting qualifies as a dramatic treatment. The patient can see something being done, and can feel something being done (including the effects of the loss of blood - the point at which this is felt is often the point at which the bleeding is stopped).

To avoid this problem, it is important for test of the effectiveness of a treatment to be conducted as double-blind trials, where the control group is given a placebo treatment, and neither the subjects nor those giving them their treatment or placebo know who is in the treatment group or control group.

For further reading on the second and third points above, see

Fourth, bloodletting was supported by the best medical theories of the time. The Western version was the four humours: blood, yellow bile, black bile, and phlegm. Other versions were expressed as the balance between hot and cold, wet and dry. Indeed, the four humours were described as combinations of hot/cold and wet/dry:

and identified with the four elements (earth, air, fire, water). The basic theory is that disease (or at least some diseases) are caused by an imbalance between the humours, or an imbalance between hot/cold or wet/dry. A variety of treatments were based on this type of theory, and some of them worked (even without the placebo effect), and caused no harm to patients. Notably, diet was used as a way to adjust the level of the humours or the hot/cold, wet/dry balance. Bloodletting was simply a direct way of adjusting the balance between the humours. When doctors (and patients) observed that it worked due to the first three reasons above, this was simply what they expected from medical theory and saw no need to investigate further. Purging (by giving laxative or emetics) was also used to restore the balance between the humours, and for the same three reasons above, would easily be believed to be effective.

A broader range of theories exist that support the effectiveness of bloodletting. For example, if disease is thought to result from "bad blood", then getting rid of some of that bad blood will help the patient. As long as there is a theoretical basis for bloodletting, the apparent effectiveness is unlikely to be questioned, and doctors and patients will continue to believe that it works, and continue to use it. There is a certain amount of circularity here: because the treatment is observed to work, belief in the theory is reinforced. In turn, this reinforces belief in the treatment.

Further further reading: