Post hoc ergo propter hoc – the cause and effect logical fallacy

This article will focus on the informal logical fallacy of post hoc ergo propter hoc (see Note 1), or post hoc fallacy. This fallacy is often used to support the use of anecdotal evidence, which has little validity in scientific discussions.


Post hoc ergo propter hoc is a Latin phrase, literally translated as “after this, therefore because of this.” Essentially, the post hoc fallacy is an argument that states that because a second event follows the first, the first event must be the cause of the second.

Many superstitions are based on this type of argument, because the observer may notice that performing one action seems to lead to another. For example, someone may have seen a black cat before someone died, and illogically applied causality to the appearance of that black cats.

The Vaccine Adverse Event Reporting System (VAERS) is an important example of the post hoc fallacy. The VAERS system allows individuals to report adverse events after vaccines. But the VAERS system has many issues, from manipulation resulting from false reports to lack of medical investigation after reporting.

Most importantly, the reporting has a preponderance of the post hoc ergo propter hoc fallacies. Almost every single report has not and, probably cannot,  show causality between the vaccine event and the adverse event. Many of the reports lack temporal correlation (let alone causation).

However, post hoc ergo propter hoc observations could lead to a testable hypothesis as long as there are certain conditions met, such as biological plausibility and potential temporal relationship. There must be robust evidence of a mechanism that extends the correlation to causation. If one reads papers (beyond the paragraph abstract) that attempt to show causality from epidemiological studies, those papers devote a section describing plausible mechanisms that support the causality hypothesis.

On the other hand, many of the claims made in VAERS are simply biologically or temporally implausible.

Throughout the pseudoscientific world, the post hoc fallacy is a fundamental foundation of their “evidence.” Homeopathic potions, which are essentially pure water, and many other alternative medicine treatments use a lot of anecdotal testimonies that are based on the post hoc ergo propter hoc fallacy.

Finally, the statement that “correlation does not imply causation” is related to the post hoc fallacy. Showing causation requires additional research far beyond showing correlation.


Ultra right winger, Andrew Schlafly, once wrote this about abortion: “In Romania, abortion was illegal under two decades of rule by the communist dictator Nicolae Ceausescu, and the country enjoyed one of the lowest breast cancer rates in the entire world during that time, far lower than comparable Western countries.”

There is no credible evidence that a lower rate of abortion leads to a lower risk of breast cancer. And this type of correlation is not evidence of causation. A proper case control epidemiological study examines the possibility of confounding factors that may have been the actual impact on the breast cancer risk. Maybe smoking rates dropped. Or a change in diet. Or anything.

Bad science relies on the post hoc fallacy.


  1. I generally italicize latin words. However, according to both the Associate Press Stylebook and various scientific stylebooks, italicization of latin words has been deprecated, especially if they are used commonly. There are a few exceptions, like scientific words like in vivo, in vitro, or in situ because the two words together having meaning. We learn something new every day.

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