Post hoc ergo propter hoc

Often I receive useful and interesting reactions to my blogs and I am my readers grateful for them. One such a reaction was by a reader of my blog last week, who added the comment “post hoc ergo propter hoc!”, to which I reacted “Exactly!”, for that is what my argument is (see note). I concluded seemingly that the recent cold waves in Europe and elsewhere in the world had been caused by the Climate Conference in Copenhagen. In fact, my blog was a cynical critique of the Climate Conference, of course, and I suppose that the readers of my blog have understood this.
Usually post hoc ergo propter hoc arguments and related reasonings like cum hoc ergo propter hoc (see note) are not as innocent as in my blog last week. We hear them everywhere around us and we see everywhere that people behave as if such arguments are valid, with the most serious consequences. For instance, the increase of crime is often ascribed to the rising numbers of foreigners in a community without any clear evidence. But maybe the truth is that the crime rate has risen because the foreigners are assaulted by the autochthonous people. Or another one, the economy is going down and the unemployment increases. What one often hears then is: “Those foreigners did it; they have taken our jobs”. The fact is, however, in many cases, that “those foreigners” have jobs that most local people do not want to do and the economy would even be more in trouble if there were no foreign labourers to do that work. And what to think of the holocaust? Jews were murdered because they were used as scapegoats for who knows what. The murder of Tutsi by Hutu in Rwanda is another case in point. Often we see here also the false argument that facts are ascribed to individuals because they belong to a group. Suppose that 60% of the Bytheway family are criminals. Then it is quite possible that I, the writer of this blog, am not a criminal and that I am the most honest person in the world.
What makes all these arguments so treacherous is that they might be true in the sense that it is quite well possible that what follows is caused by what precedes it or that there is another relation between things happening together. A cause precedes always its effect, for instance. And it is true, an individual might have the average characteristics of the members of the group he or she belongs to. But it is a matter of fact that such kinds of arguments are so often wrong that it is better to consider them as fallacies.

note: “Post hoc ergo propter hoc” means literally “after this, therefore because of (on account of) this”. For instance, the rooster crows, and therefore the sun rises. “Cum hoc ergo propter hoc” is the false argument saying that if two facts or events happen to take place together, one is the cause of the other. For example, in the countryside there are both more storks and the birth rate is higher there than in towns, so the higher birth rate in the countryside must have been caused by the higher number of storks. Nobody will accept such arguments in simple cases, but in more complicated cases people often do.
More on http://en.wikipedia.org/wiki/Post_hoc_ergo_propter_hoc and http://en.wikipedia.org/wiki/Correlation_does_not_imply_causation.