The Post Hoc Fallacy involves the conclusion that one event caused a second simply because the first event occurred before the second.


Latin source: post hoc, ergo propter hoc


There are 2 requirements for a post hoc fallacy:

(1) occurrence of one event prior to a second

(2) belief that the first event was causal for the second without any evidence (lack of proof)


It is not a fallacy if there is definite evidence that the second event is caused by the first.


Fallacy kicks in when no effort is made to investigate the relationship between the two events:

(1) taken as a matter of faith or superstition

(2) assumed without making any further effort

(3) refusal to consider alternative explanations

(4) continued belief despite good evidence that the first event did not cause the second


Examples of post hoc fallacy:

(1) unproven claims of adverse effects caused by vaccines

(2) unproven claims that cellular phones cause brain tumors


Limitations in determining a causal link:

• Multifactorial problems can be difficult to solve.

• An idiosyncratic reaction is hard to determine with certitude.

• Many studies involve a statistical measures which involve some degree of uncertainty.


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