Also Known as: Post Hoc Ergo Propter Hoc, False Cause, Questionable Cause,
Confusing Coincidental Relationships With Causes
A Post Hoc is a fallacy with the following form:
- A occurs before B.
- Therefore A is the cause of B.
The Post Hoc fallacy derives its name from the Latin phrase
"Post hoc, ergo propter hoc." This has been traditionally
interpreted as "After this, therefore because of this." This
fallacy is committed when it is concluded that one event causes another
simply because the proposed cause occurred before the proposed effect.
More formally, the fallacy involves concluding that A causes or caused B
because A occurs before B and there is not sufficient evidence to
actually warrant such a claim.
It is evident in many cases that the mere fact that A occurs before B
in no way indicates a causal relationship. For example, suppose Jill,
who is in London, sneezed at the exact same time an earthquake started
in California. It would clearly be irrational to arrest Jill for
starting a natural disaster, since there is no reason to suspect any
causal connection between the two events. While such cases are quite
obvious, the Post Hoc fallacy is fairly common because there are cases
in which there might be some connection between the events. For example,
a person who has her computer crash after she installs a new piece of
software would probably suspect that the software was to blame. If she
simply concluded that the software caused the crash because it was
installed before the crash she would be committing the Post Hoc fallacy.
In such cases the fallacy would be committed because the evidence
provided fails to justify acceptance of the causal claim. It is even
theoretically possible for the fallacy to be committed when A really
does cause B, provided that the "evidence" given consists only
of the claim that A occured before B. The key to the Post Hoc fallacy is
not that there is no causal connection between A and B. It is that
adequate evidence has not been provided for a claim that A causes B.
Thus, Post Hoc resembles a Hasty Generalization in that it involves
making a leap to an unwarranted conclusion. In the case of the Post Hoc
fallacy, that leap is to a causal claim instead of a general
proposition.
Not surprisingly, many superstitions are probably based on Post Hoc
reasoning. For example, suppose a person buys a good luck charm, does
well on his exam, and then concludes that the good luck charm caused him
to do well. This person would have fallen victim to the Post Hoc
fallacy. This is not to say that all "superstitions" have no
basis at all. For example, some "folk cures" have actually
been found to work.
Post Hoc fallacies are typically committed because people are simply
not careful enough when they reason. Leaping to a causal conclusion is
always easier and faster than actually investigating the phenomenon.
However, such leaps tend to land far from the truth of the matter.
Because Post Hoc fallacies are committed by drawing an unjustified
causal conclusion, the key to avoiding them is careful investigation.
While it is true that causes precede effects (outside of Star Trek,
anyways), it is not true that precedence makes something a cause of
something else. Because of this, a causal investigation should begin
with finding what occurs before the effect in question, but it should
not end there.
- I had been doing pretty poorly this season. Then my
girlfriend gave me this neon laces for my spikes and I won my next
three races. Those laces must be good luck...if I keep on wearing
them I can't help but win!
- Bill purchases a new PowerMac and it works fine for months.
He then buys and installs a new piece of software. The next time he
starts up his Mac, it freezes. Bill concludes that the software must
be the cause of the freeze.
- Joan is scratched by a cat while visiting her friend. Two
days later she comes down with a fever. Joan concludes that the
cat's scratch must be the cause of her illness.
- The Republicans pass a new tax reform law that benefits
wealthly Americans. Shortly thereafter the economy takes a nose
dive. The Democrats claim that the the tax reform caused the
economic woes and they push to get rid of it.
- The picture on Jim's old TV set goes out of focus. Jim goes
over and strikes the TV soundly on the side and the picture goes
back into focus. Jim tells his friend that hitting the TV fixed it.
- Jane gets a rather large wart on her finger. Based on a story
her father told her, she cuts a potato in half, rubs it on the wart
and then buries it under the light of a full moon. Over the next
month her wart shrinks and eventually vanishes. Jane writes her
father to tell him how right he was about the cure.
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