Late February, early March, in Europe, storks are back on the chimneys! This year again, they will bring beautiful babies into homes…

Finally, this is what children are told, because no one believes in this cute legend. But by the way, where does it come from?

Weddings are often held in early summer, in June-July. And in earlier times, almost elsewhere, children were often conceived immediately after the wedding, especially because there was no effective means of contraception. After 9 months of pregnancy, a peak of births happened around March, precisely when the storks returned from their winter migration!

That’s where this little story comes from, the observable link between births and the return of storks. It is a correlation, namely a statistical relationship between two phenomena. But that doesn’t mean one is caused by the other. That’s causality, and we must not confuse correlation and causality as the legend does!

In economics, for example, looking at a graph, one might think that the more mobile phones a country is equipped with, the less infant mortality there is. The two elements are correlated, that’s for sure, but the real cause of the link lies elsewhere: the country concerned is rich enough to have good maternity wards and a population that can afford mobile phones.

Between storks and births, it’s the same thing. Causality is simply the calendar, which makes the reproductive habits of humans coincide with the migration of storks. This cute legend thus originates from an error of reasoning of which we are all victims and that we call the stork effect!

Did you know this little story about storks?