Testimonial evidence exists for pretty much any claim that has ever been devised — alien abductions, demon possessions, miracle medical cures and the like.

One needs to look no further than the dietary supplement industry to see the influence of testimonials. In fact, testimonials are probably the key marketing tool for the supplement industry. Medicine, psychology, and the beauty industry, to name a few, often refer to testimonials in an effort to show the efficacy of their products or treatments. It is not uncommon for people to make decisions based on testimonials that conflict with scientific evidence — giving more weight to the testimonial.

This is a mistake because testimonials are not real evidence.

Placebo Effect

“Placebo” is derived from a Latin word meaning “I shall please.” It has been known for some time that the mere expectation of improvement leads to improvement. The placebo effect occurs when people report their condition has improved after receiving any treatment, regardless of its therapeutic value. The power of the placebo effect is illustrated in the movie classic, The Wizard Of Oz. The wizard didn’t actually give the scarecrow a brain, the tin man a heart, and the lion courage, but they all felt better anyway (Stanovich, 2007).

It can be expected that the benefits obtained from any treatment are partly due to placebo effects. “[S]ubjects typically know they are getting some kind of treatment, and so we may rarely be able to measure the actual effects of a drug by itself. Instead, we see the effects of treatment plus placebo effects that are shaped by the subjects’ expectations. We then compare those effects with the effects of placebo alone” (Myers and Hansen, 2002).

Generally, when conducting studies on a new drug a group is given the experimental drug while another equivalent group (control group) is given a placebo, an inert substance which does not contain the drug. The results of the two groups are then compared. Without the use of a control group it would be impossible to know what percentage of people report benefits due to receiving the drug rather than benefits due to a placebo effect.

Vividness Effects

Making decisions based on testimonials can be dangerous. Compelling personal testimonials often dissuade people from accepting scientific evidence. The vividness of personal testimony often trumps evidence of higher reliability. Psychologists call this problem in belief formation the vividness effect (Stanovich, 2007).

Society is replete with examples of the vividness effect. To further illustrate this point consider the following scenario. You are deciding whether you should try a dietary supplement that is purported to decrease appetite. After reading the scientific research on the product you conclude that the supplement does not decrease appetite. The next day you mention the supplement to your friend, who suggests the supplement worked great for her.

Should this anecdote persuade you to purchase the supplement, even though scientific data suggests different? There is a good chance that the friend’s testimony would outweigh the scientific evidence. The vividness effect is widespread and often leads to bad decisions (purchasing worthless drugs, supplements, dietary programs, not vaccinating children, etc.).

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Testimonials are easy to generate and have been produced for all sorts of claims. However, testimonials should never be confused with scientific evidence — or portrayed in a way suggesting they are equivalent. Testimonials may provide ideas that warrant further investigation, but that’s it.