What is Post hoc ergo propter hoc?

The Latin translates as ‘after this, therefore on account of this’, and it is the fallacy of supposing that because one event follows another, then the second has been caused by the first.


Immediately after the introduction of canned peas, the illegitimate birthrate shot up to a new high from which it did not decline until frozen peas edged canned peas out of the market. The link is all too obvious.

(Too obvious to be true, perhaps. If your thoughts turn to feeding your daughters beans instead, remember to keep them clear of everything else which preceded the rise in illegitimacy. They should stay away from television, jet aircraft, polythene and chewing-gum, to name but a few of the more obvious hazards.)


Although two events might be consecutive, we cannot simply assume that the one would not have occurred without the other. The second might have happened anyway. The two events might both be linked by a factor common to both. Increased prosperity might influence our propensity to consume canned peas, and also to engage in activities which increase the rate of illegitimacy. Small children at gaming machines provide vivid illustrations of the post hoc fallacy. They may often be seen with crossed fingers, eyes closed, hopping on one leg, or in whatever physical contortion once preceded a win. They link their random preparations with the outcome of their luck; and in this they differ in no wise from more adult gamblers, whose concealed rabbits’ feet and clenched-teeth incantations betray the same supposition. If it worked once, it can work again.

Unfortunately for our predictive ability, every event is preceded by an infinite number of other events. Before we can assign the idea of cause, we need rather more than simple succession in time. The philosopher David Hume pointed to regularity as the chief requirement, with some contiguity in time and space. We are more likely to describe a germ as the cause of an infection in a man if its presence has regularly preceded the infection, and if it is found in the body which is infected.

The charm of the post hoc fallacy emerges when we leave behind the everyday idea of cause and effect. Although we suppose we understand the mechanisms by which one event leads to another, Hume showed that it boils down to our expectation of regularity. The candle flame on the finger and the subsequent pain are called cause and effect because we expect the one to follow on regularly from the other. Of course, we concoct all kinds of explanations as invisible threads to link the two, but they come down to interposing unseen events between our first and second. How do we know that these unseen events really are the cause? Easy. They always follow from one another. This gap in our knowledge provides a vacant lot in which fallacies can park at will. Greek historians regularly discussed natural disasters in terms of human actions. In looking for the cause of an earthquake, for example, we are likely to find Herodotus, or even Thucydides, gravely discussing the events which preceded it before concluding that a massacre perpetrated by the inhabitants of the stricken town was probably the cause.

The determined fallacist will see this as a field of opportunity. Whatever your opponent is urging is bound to have been tried somewhere, in some form, at some time. All you need do is attribute the unpleasant things which followed it to the operation of that factor. We know that unpleasant things followed it because unpleasant things are happening all the time; there are always plenty of earthquakes, sex offences and political broad-casts on TV which you can lay at your adversary’s door.


‘Imprisonment is barbaric. We should try to understand criminals and cure them using open prisons and occupational therapy. ‘

‘They have been trying that in Sweden since 1955, and look what happened: suicides, moral degeneracy, and drunks everywhere. Do want that here?’

(A term such as ‘moral degeneracy’ is the hallmark of the sterling fallacist, being more or less impossible to disprove.)