December 18, 2012


ECONOMIST 25 St. James Street
London SW1A 1HG

You wisely point out the seriousness of the double demographic whammy – falling immigration and falling fertility – striking the US1 attributing both to economic causes.  In the case of migration I imagine you are right, but in the case of fertility you have all inadvertently said, as the houyhnhnms might have phrased it, that which is not.  A couple of studies done in Denmark2,3 found that once consanguinity effects were excluded by controlling for the size of the community a couple lived in and the distance between their birthplaces there was no effect of income or education upon fertility.  There was a zone of consanguinity with greatest fertility with less fertility above and below the zone.  Similarly in Iceland4 there was the same relationship between consanguinity and fertility with error bars so tight they left essentially no room for any economic influence.  The team found the same pattern (except for the absence of inbreeding depression in the first generation) in grandchildren as well as in children.  The same effect has been seen in over a thousand studies in animals5.  A more recent study published by the Royal Society6 shows that in the third generation after being rich (which I take to be a surrogate for a dramatic reduction in consanguinity) a couple takes an even more severe loss in great grandchildren. 

There is no need to extrapolate.  History is quite clear on what happens.  Civilizations go down the siphon.  It isn’t pretty to watch.  For the first time we have good demographic statistics although in ancient Greece and Rome reduced fertility was remarked upon. 

1Double Bind ECONOMIST vol. 405 no. 8815 December 15. 2012 page 31
2Rodrigo Labouriau and António Amorim Human Fertility Increases with marital radius GENETICS volume 178 January 2008 page 603
3 Rodrigo Labouriau and António Amorim Comment on “An Association Between the Kinship and Fertility of Human Couples,” SCIENCE vol. 322, page 1634b December 12, 2008
4Agnar Helgason et al. An Association between Kinship and Fertility of Human Couples SCIENCE vol. 329 no. 5864 February 8, 2008 page 813 – 816
5Richard M. Sibly et al. On the Regulation of Populations of Mammals, Birds, Fish and Insects SCIENCE vol. 309 July 22, 2005 page 609 
6Anna Goodman et al. Low fertility increases descendant socioeconomic position but reduces long-term fitness in a modern post-industrial society PROCEEDINGS OF THE ROYAL SOCIETY B (Proc. R. Soc. B 2012 279, 4342-4351)


M. Linton Herbert MD


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