Archive for July, 2020

“This” Happened After “That,” So “This” Caused “That”: Not So Fast!

July 25, 2020

Andrew Siegel MD    7/25/2020


One of my truly extraordinary professors during my urology residency at University of Pennsylvania–a pediatric urologist at Children’s Hospital of Philadelphia who recently passed away–was legendary for his concise statements of scientific principles that he often expounded in the operating room.  One of his favorite ones was “Post hoc ergo proctor hoc,” translated from Latin as: “After this therefore because of this,” a fallacy that easily lulls one down the wrong thought pathway, misleading and deceiving.  This failure of logic is commonly committed in many domains of life, particularly so in medicine. 

Post hoc ergo proctor hoc” is a fallacy that essentially states that since event Y followed event X, event Y must have been caused by event X. This misinterpretation that falsely causally relates two events that follow in time sequence is often referred to more concisely as the “post hoc” fallacy.  The bottom line is that just because something follows something else in time sequence, one cannot assume that they are causally related.

You know the saying about ASSUMING: “Never ASSUME, because when you ASSUME, you make an ASS out of U and ME.”



Karen Arnold has released this “Rooster Silhouette Crowing Sunrise” image under Public Domain license.

A classic example of the post hoc fallacy is the following: After the rooster crows, the sun rises– therefore, the rooster crowing causes the sun to rise.  POST HOC ERGO PROPTER HOC!


It is easy to be deceived by the post hoc fallacy, as I was recently, in a situation that provides a subtler example than the sunrise and the rooster.  In the midst of a telemedicine consult with a physician patient, I received a text from his son, also a physician, requesting a phone call.  Naturally, I assumed the son’s text was related to his father’s medical situation.  I phoned the son immediately after the telemedicine consult and as it turned out he had no idea that his father and I had a telemedicine consultation. The reason for the son’s call was totally unrelated, but he was quite relieved to hear that his dad was doing well! POST HOC ERGO PROPTER HOC!

Here’s another one: I returned to my office on Monday morning following the July 4th holiday weekend and found that I had left my iPod on (yep, I still have one!) playing music in the Bose docking station/speaker that is plugged into a power strip. When I went to charge my MacBook using the power cord that I leave plugged into the same power strip that the Bose is plugged into, I noted that the power cord was not working for the first time ever.  My assumption was that leaving the iPod on for almost 4 days somehow damaged the MacBook power cord.  POST HOC ERGO PROPTER HOC!


True, True—Unrelated” is another way of stating the post hoc fallacy.  My brother-in-law was infected with COVID-19 in February and after a course of hydroxychloroquine along with a Z-pack the infection ultimately resolved and therefore he concluded that the infection was cured by the medications.  However, studies have clearly shown that these medications offer no benefit. True he had COVID-19, True he took the medications and  the infection resolved,  but these events were unrelated.  POST HOC ERGO PROPTER HOC!


There are many unregulated (not FDA approved), over-the-counter, “natural” supplements that people readily purchase and use and when their situation improves (as it often does spontaneously) the improvement is attributed to the supplement. While this may be the case, the much more likely possibility is that nature and time resolved the issue.  The placebo effect is a powerful force that contributes to the belief in this fallacy.  POST HOC ERGO PROPTER HOC!


It is human nature for physicians to attribute good results following a course of treatment to the treatment. Fortunately, this is often the case, but ignores the equally plausible possibility that the good results are a consequence of the natural healing process and not the treatment. POST HOC ERGO PROPTER HOC!

The best fortune cookie I ever received stated: Nature, Time and Patience are the 3 great physicians.  SPOT ON!

By the same token, some patients following best practice, state-of-art-treatments have poor results. It is common to attribute the poor results to the treatment, but this ignores the equally plausible possibility that the poor results are the results of the natural history of the disease. POST HOC ERGO PROPTER HOC!

Wishing you the best of health,

2014-04-23 20:16:29


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Dr. Andrew Siegel is a physician and urological surgeon who is board-certified in urology as well as in female pelvic medicine and reconstructive surgery.  He is an Assistant Clinical Professor of Surgery at the Rutgers-New Jersey Medical School and is a Castle Connolly Top Doctor New York Metro Area, Inside Jersey Top Doctor and Inside Jersey Top Doctor for Women’s Health. His mission is to “bridge the gap” between the public and the medical community. He is a urologist at New Jersey Urology, the largest urology practice in the United States.  His latest book is Prostate Cancer 20/20: A Practical Guide to Understanding Management Options for Patients and Their Families. 

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Video trailer for Prostate Cancer 20/20

Preview of Prostate Cancer 20/20

Andrew Siegel MD Amazon author page

Prostate Cancer 20/20 on Apple iBooks

PROSTATE CANCER 20/20: A Practical Guide to Understanding Management Options for Patients and Their Families is now on sale at Audible, iTunes and Amazon as an audiobook read by the author (just over 6 hours). 

Dr. Siegel’s other books:

FINDING YOUR OWN FOUNTAIN OF YOUTH: The Essential Guide to Maximizing Health, Wellness, Fitness and Longevity

PROMISCUOUS EATING— Understanding and Ending Our Self-Destructive Relationship with Food

MALE PELVIC FITNESS: Optimizing Sexual and Urinary Health

THE KEGEL FIX: Recharging Female Pelvic, Sexual, and Urinary Health