Alias: Post Hoc, Ergo Propter Hoc
Translation: After this, therefore because of this, Latin
Type: Non Causa Pro Causa
Roosters crow just before the sun rises.
The Post Hoc Fallacy is committed whenever one reasons to a causal conclusion based solely on the supposed cause preceding its "effect". Of course, it is a necessary condition of causation that the cause precede the effect, but it is not a sufficient condition. Thus, post hoc evidence may suggest the hypothesis of a causal relationship, which then requires further testing, but it is never sufficient evidence on its own.
Post Hoc also manifests itself as a bias towards jumping to conclusions based upon coincidences. Superstition and magical thinking include Post Hoc thinking; for instance, when a sick person is treated by a witch doctor, or a faith healer, and becomes better afterward, superstitious people conclude that the spell or prayer was effective. Since most illnesses will go away on their own eventually, any treatment will seem effective by Post Hoc thinking. This is why it is so important to test proposed remedies carefully, rather than jumping to conclusions based upon anecdotal evidence.
Sibling Fallacy: Cum Hoc, Ergo Propter Hoc
T. Edward Damer, Attacking Faulty Reasoning: A Practical Guide to Fallacy-Free Arguments (Third Edition) (Wadsworth, 1995), pp. 131-132.
These two examples show how the same fallacy is often exploited by opposite sides in a debate, in this case, the gun control debate. There are clear claims of causal relationships in these arguments. In the anti-gun control example, it is claimed that so-called "right-to-carry" laws "effectively reduce" public shootings and violent crime. This claim is supported by statistics on falling crime rates since the mid-1980s in states that have passed such laws. In the pro-gun control example, it is claimed that state and local gun control laws "work", presumably meaning that the laws play a causal role in lowering handgun crime. Again, the claim is supported by statistics on falling crime rates in one state. However, the evidence in neither case is sufficient to support the causal conclusion.
For instance, violent crime in general fell in the United States in the period from the mid-1980s to the present, andfor all that we can tell from the anti-gun control argumentit may have fallen at the same or higher rates in states that did not pass "right-to-carry" laws. Since the argument does not supply us with figures for the states without such laws, we cannot do the comparison.
Similarly, the pro-gun control argument does not make it clear when Massachusett's drop in crime occurred, except that it was "after""post hoc"the handgun control law was passed. Also, comparative evidence of crime rates over the same period in states that did not pass such a law is missing. The very fact that comparative information is not supplied in each argument is suspicious, since it suggests that it would have weakened the case.
Another point raised by these examples is the use of misleadingly precise numbers, specifically, "7.65%" and "5.2%" in the anti-gun control example. Especially in social science studies, percentage precision to the second decimal place is meaningless, since it is well within the margin of error on such measurements. It is a typical tactic of pseudo-scientific argumentation to use overly-precise numbers in an attempt to impress and intimidate the audience. A real scientist would not use such bogus numbers, which casts doubt upon the status of the source in the example. The pro-gun control argument, to its credit, does not commit this fallacy. This suggests, though it doesn't nail down, an appeal to misleading authority in the anti-gun control one.