There are times when you read a paper and you really wonder how the authors came up with the idea to check out a particular correlation in the data, because it seems to be so far-fetched. But thus a correlation can be beautiful if it also has a nice theory that comes with it.
The correlation that Alberto Alesina, Paola Giuliano and Nathan Nunn study is between current fertility and adoption of plough agriculture in history. OK, I did not think about that one. But now that they find a nice positive correlation, how could one explain it? They argue that this has to do that women and children are not particularly useful when ploughing, as strength is required. The traditional task of weeding, that fell on women and children, is not necessary with ploughing. Thus, there is a preference for fewer children that is ingrained in the culture of these regions to this day.