low fertility rates ’cause we don’t marry our cousins

i was going to write a lengthy and very informative post about the likelihood that low fertility rates in our modern world might be related to the fact that we don’t marry our cousins very much anymore (and, in the west, haven’t been for quite a long time now), but there was a good write up in the economist a couple of years ago when that research from iceland (about the fecundity of cousins) came out, so i’m just going to quote that:

“Most explanations of the demographic transition are social…..

“Now yet another explanation has been added to the pot. This is that the mixing-up of people caused by the urbanisation which normally accompanies development is, itself, partly responsible. That is because it breaks up optimal mating patterns. The demographic transition is thus, in part, a pure accident….

“This suggestion is the corollary of a paper published in this week’s Science by Agnar Helgason and his colleagues at deCODE Genetics, a firm based in Reykjavik….

The study’s principal finding is that the most fecund marriages are between distant cousins. Using Iceland’s genealogical records, which allow the degree of relatedness between husband and wife to be calculated with great precision, Dr Helgason showed the optimum degree of outbreeding (measured in terms of the number of children and grandchildren produced) lay somewhere between cousins of the third and fourth degrees….

“The strong relationship between kinship and fertility was so unexpected that the researchers have not yet calculated exactly how much it contributes to the demographic transition. But even from the figures they present, it is clearly an important factor, and one that is likely to apply in other parts of the world where the records needed to prove it are not so good. Even in poor countries, birth rates are now falling fast. An important part of the explanation may simply be the additional choice of mates that development and urbanisation bring with them.”

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