Edward Tabor
Bethesda, Maryland, United States

“The Bellman’s Fallacy” is a form of biased thinking in which something is believed to be true because it has been repeatedly stated. Its name comes from the Bellman in Lewis Carroll’s “The Hunting of the Snark,” who says, “What I tell you three times is true.”¹ Based on this poem, the phrase itself was used for many years in non-medical contexts to refer to this type of thinking. (President Theodore Roosevelt occasionally quoted it.²) It was given the name “The Bellman’s Fallacy” when it was first applied to medical information in 1973.^3,4 Today, the proliferation of medical articles on the internet increases the possibility that this type of faulty thinking will occur.

The term was first used in the medical literature to refer to the incorrect claim that Hippocrates first described lead poisoning, which was reported by several authors who were quoting each other.⁴ Later the term was applied to an incorrect statement that undertreatment was the cause of an epidemic of asthma deaths, based on its having been discussed in three earlier publications.⁵

The internet has facilitated the wider occurrence of this potential bias. Many medical headlines and unusual articles result in spin-off articles in countless online publications intended either for medical readers or patients. They turn up in a Google search and sometimes are accepted as true on the basis of multiple citations.

An example of the internet effect occurred in 2023 in a review article by Sahni and Carrus.⁶ The authors stated that the growth of “the collective body of medical knowledge required to treat a patient” had a doubling time in 2010 of less than seventy-five days and “today, what medical students learn in their first 3 years would be only 6 per cent of known medical information at the time of their graduation.” The doubling time was referenced to an article by Densen,⁷ who stated a doubling time of seventy-three days (for 2020, not 2010) and also mentioned the 6% figure, but provided no evidence for either.

A Google search for “doubling time of medical information”⁸ found twenty relevant citations, nineteen of which cited Densen’s 73-day figure; of these, fourteen referenced the Densen paper and five cited ”73 days” without attribution. Thus, the one article by Densen,⁷ stating numbers with no evidence basis, was responsible for nineteen Google citations and a further restatement in the Sahni and Carrus review in a prominent journal, making it even more likely to continue being repeated.

It is a normal part of human cognition to accept statements as true when they have been heard repeatedly, but an alert reader must try to avoid doing so without first verifying that the statements are based on data. Familiarity due to having seen something stated multiple times is not easily distinguished from truth because of the “cognitive ease” it provides.⁹ Avoiding “The Bellman’s Fallacy” has become more difficult in the internet age, but avoiding it should be a goal of every physician reading the medical literature.

References

1. Gardiner, M., editor: Carroll, L. “The Hunting of the Snark.” The Annotated Snark, Penguin Books, Harmondsworth, UK, 1962, page 46. First published 1876.
2. Ibid., pages 46-47, footnote 7.
3. Skrabanek P, McCormick J. Follies and Fallacies in Medicine. Prometheus Books, Buffalo, NY,1990, pages 36-37.
4. Waldron H.A. “Hippocrates and lead.” Lancet 1973;2:626 (letter).
5. Stolley P.D., Lasky T. “The Bellman always rings thrice.” Ann. Int. Med. 1993;118:158 (letter).
6. Sahni NR and Carrus B. “Artificial intelligence in U.S. health care delivery.” N Engl J Med 2023;389:348-358
7. Densen P. Challenges and opportunities facing medical education. Trans Am Clin Climatol Assoc 2011;122:48-58.
8. Google search performed on August 24, 2023.
9. Kahneman D. Thinking, Fast and Slow. Farrar, Straus and Giroux, New York, 2011, page 62.