What is Cum hoc ergo propter hoc?

The cum hoc fallacy assumes that events which occur together are causally connected, and leaves no room either for coincidence, or for the operation of an outside factor which separately influences those events.


A tourist met a Spanish peasant and his wife on a train. They had never seen bananas before*, so he offered one to each of them. As the farmer bit into his, the train entered a tunnel. ‘Don’t eat it, Carmen/ he shouted, ‘They make you blind.’


Like the post hoc fallacy which links events because they occur consecutively, the cum hoc fallacy links them because they occur simultaneously. It is a fallacy because of its unwarranted assumption that either of the events would not occur without the other one.

Things are happening all the time. Scarcely a day goes by without rain, electricity bills, televised show-jumping, or the Guardian newspaper. It is inviting to link these endemic discomforts with simultaneous events, and conclude that they are all somehow connected. In primitive societies such assumptions are made routinely, and one of the jobs of the witchdoctor is to sort out which actions are linked with various consequences. In our society, alas, life is more complicated.

The field of statistics provides a natural habitat for the cum hoc fallacy to lurk undetected. Indeed, a whole branch of statistics called regression analysis is devoted to measuring the frequency and extent of simultaneous events, and to calculating the probability of their being linked. Correlation coefficients are produced, with percentages attached showing the likelihood that mere chance was involved. Statisticians routinely offer us relationships with a 95 per cent or a 99 per cent probability that ‘more than chance is involved’.


A statistician looking over figures for pupil performance was astounded to discover in the 7-12 age-group that neatness of handwriting matched with size of shoe. He checked the figures for hundreds of children, but it was quite clear. Neat handwriting correlated with large feet, with 99 per cent probability that this was not mere chance.

(A teacher later told him that this was because older children tended to write more neatly. Being older, they tended to have bigger feet.)


Most disciplines which involve human measurement, including economics and sociology, find cum hoes scattered liberally on their domain. The reason for this is that we do not really know what makes human beings act, so we look at their actions after the fact and try to relate them to other events. The cum hoc tares grow up with the wheat of genuine insights.


Elections make people spend. The figures are clear. Spending always goes up in an election year.

(Could it be that governments seeking re-election tend to keep taxes down in election years, and that people, in consequence, have more to spend?)


Deliberate use of the cum hoc ergo propter hoc is best made with the support of reams of statistical information. Your audience, bemused by the figures, rarely have any of their own to set against you. They can be made even more disposed to accept the link which you are proposing if you cite the authority of leading figures in the social sciences. This is easy. There is nothing so absurd that it has not been attested to by such people. It helps to be selective in your use of information.


Gun ownership is a major cause of violent crime. The prevalence of guns in the US matches the high rates for crimes of violence. When violence is contemplated, the guns are all too available.

(Excellent; but remember not to mention Switzerland, where almost every household has a gun as part of military training. Switzerland has low rates for violent crime, and the guns are almost never used.)


A US legislator recently noted that a high crime-rate correlated with a high prison population, and suggested that the prisoners be released in order to cut the crime figures.

For use of the fallacy in print, simply juxtapose articles. Study the front pages to see how it is done.




The above article is from the book How To Win Every Argument by Madsen Pirie. The article is only for educational and informative purposes to explain and understand formal logic and logical fallacies. It is a great book, definitely worth a read!