What Is a Post Hoc Logical Fallacy?

Man sitting on rock with picnic baskets
Eula Becker's presence at the picnic didn't cause the rain. Johner Images/Getty Images

Post hoc (a shortened form of post hoc, ergo propter hoc) is a logical fallacy in which one event is said to be the cause of a later event simply because it occurred earlier. "Although two events might be consecutive," says Madsen Pirie in "How to Win Every Argument," "we cannot simply assume that the one would not have occurred without the other."

Why Post Hoc Is a Fallacy

Post hoc is a fallacy because correlation does not equal causation. You cannot blame your friends for a rain delay just because every time they go with you to a ballgame it storms and play is delayed. Likewise, the fact that a pitcher bought new socks before he pitched a winning game does not mean that new socks cause a pitcher to throw faster.

The Latin expression post hoc, ergo propter hoc can be translated literally as "after this, therefore because of this." The concept can also be called faulty causation, the fallacy of false cause, arguing from succession alone or assumed causation.

Post Hoc Examples: Medicine

The search for causes of diseases is rife with post hoc examples. Not only are medical researchers constantly seeking causes of or cures for medical maladies, but patients are also on the lookout for anything—no matter how unlikely—that might help to alleviate their symptoms. In some cases, there is also a desire to find a cause outside of genetics or luck that can be blamed for health or developmental challenges.

Malaria

The long search for the cause of malaria was fraught with post hoc fallacies. "It was observed that persons who went out at night often developed the malady. So, on the best post hoc reasoning, night air was assumed to be the cause of malaria, and elaborate precautions were taken to shut it out of sleeping quarters," explained author Stuart Chase in "Guides to Straight Thinking." "Some scientists, however, were skeptical of this theory. A long series of experiments eventually proved that malaria was caused by the bite of the anopheles mosquito. Night air entered the picture only because mosquitoes preferred to attack in the dark."

Autism

During the early 2000s, the search for a cause of autism led to vaccines, though no scientific link has been found between the administration of vaccines and the onset of autism. The time that children are vaccinated and the time they're diagnosed do closely correlate, however, leading upset parents to assign blame to the immunizations, for lack of a better explanation.  

Post Hoc Variation: Inflated Causality

In the inflated causality version of post hoc, the proposed idea tries to boil down a happening to one singular cause, when in actuality, the event is more complex than that. However, the idea is not completely untrue, which is why it's called inflated rather than just completely faulty. For example, each of these explanations is incomplete:

  • Attributing the cause of World War II to only Adolf Hitler's hatred of the Jews
  • Suggesting that John F. Kennedy won the presidency over Richard Nixon exclusively because of the debate on TV
  • Believing that the cause of the Reformation was simply Martin Luther posting his theses
  • Explaining that the U.S. Civil War was fought only because of the institution of slavery

Economics is a complex issue, so it can be a fallacy to attribute any particular happening to just one cause, whether it be the latest unemployment statistics or one policy being the magic fuel for economic growth.

Post Hoc Examples: Crime

In a search for reasons for increased crime, a "New York Times" article by Sewell Chan entitled "Are iPods to Blame for Rising Crime?" September 27, 2007) looked at a report that appeared to blame iPods:

"The report suggests that 'the rise in violent offending and the explosion in the sales of iPods and other portable media devices is more than coincidental,' and asks, rather provocatively, 'Is There an iCrime Wave?' The report notes that nationally, violent crime fell every year from 1993 to 2004, before rising in 2005 and 2006, just as 'America’s streets filled with millions of people visibly wearing, and being distracted by, expensive electronic gear.' Of course, as any social scientist will tell you, correlation and causation are not the same thing."

Sources

  • Chan, Sewell. “Are IPods to Blame for Rising Crime?” The New York Times, The New York Times, 27 Sept. 2007, cityroom.blogs.nytimes.com/2007/09/27/are-ipods-to-blame-for-rising-crime/.
  • Chase, Stuart. Guides to Straight Thinking. Phoenix House, 1959.
  • Pirie, Madsen. How to Win Every Argument: the Use and Abuse of Logic. Continuum, 2016.