Today’s Lesson in Logic

From Boing-Boing, some guy named Mark Frauenfelder brings us a brilliant example of the logical fallacy known as Post hoc, ergo propter hoc:

Fraunfelder jokes, based on the pictures, “It’s conclusive: owning a passport will prevent you from becoming diabetic.” 

More exactly, this is the related fallacy with the latin name Cum hoc, ergo propter hoc. Those latin phrases mean “After/with this, therefore because of this.” Both of those fallacies fall in the category of Non causa, pro causa (“Non-cause for cause”). When two effects are seen, one fallaciously assumes that one effect caused the other. In reality, logic allows that the two effects are both caused by other, common causes — in this case, probably things like poverty, education, ethnic diversity, etc.

Another correct way to approach this kind of data, is to remember the statistical truth, “correlation does not imply causation.” In this case, populations that are more likely to have passports, are also more likely to have diabetes — so passport-ownership and diabetes are correlated. But that doesn’t mean that passports cause diabetes (any more than getting diabetes will get you a passport)!


2 Responses

  1. I agree with your assessment that if two facts or statistics are correlated that doesn’t necessarily mean causation. However, it looks like in these two instances, owning a passport and having diabetes are inversely correlated. So the logical fallacy here is that owning a passport would prevent diabetes. In any case the map for diabetes makes sense that diabetes rates are higher in the South where diets tend to be very non-healthy and lower in the West and Northeast where people tend to eat healthier and exercise more.

  2. From my favorite informal fallacies guru, Douglas Walton

    Another fallacy that fits very well under this category is that of post hoc reasoning, often called “false cause”. Arguing from a perceived correlation between two events to the conclusion that one causes the other is, in principle, a legitimate form of reasoning. Indeed, many inductive arguments to causal conclusions are based on correlations. The fallacy of post hoc is said to arise when the arguer jumps too quickly to the causal conclusion, while overlooking other evidence that ought to be taken into account, and that would indicate that reservations need to be considered.

    Read the rest here

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