Post Hoc: Definition and Examples of the 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 fallacy in which one event is said to be the cause of a later event simply because it occurred earlier. Remember, 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, because a baseball pitcher grows a beard (or wears the same socks, etc.) during the postseason doesn't mean he will pitch better.

"Although two events might be consecutive," says Madsen Pirie in "How to Win Every Argument" (2015), "we cannot simply assume that the one would not have occurred without the other."

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. Take the long search for the cause of malaria. "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. "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." ("Guides to Straight Thinking." Harper, 1956)

The search for the cause of autism and placing blame on vaccines could also be a post hoc fallacy, as 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, John F. Kennedy winning the presidency over Richard Nixon because of the debate on TV, the cause of the Reformation being Martin Luther posting his theses, or the U.S. Civil War being fought because of slavery. 

Economics is a complex issue, with many aspects of it running concurrently, especially when you're talking about a nation's economy. 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 looked at a report that appeared to blame iPods:

"The Urban Institute, a research organization based in Washington, has released an interesting report that suggests that the proliferation of iPods helps account for the nationwide rise in violent crime in 2005 and 2006.
"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."
(Sewell Chan, "Are iPods to Blame for Rising Crime?" September 27, 2007)