June 26, 2013
to be posted on nobabies.net

Dear Abraham Bruunk,

I have read the abstract and been able to skim your interesting article. 

(A slow life history is related to a negative attitude towards cousin marriages: A study in three ethnic groups in Mexico

Evolutionary Psychology 11(2): 442-458 Abraham P. Buunk, Royal Netherlands Academy of Arts and Sciences and Department of Psychology, University of Groningen, Groningen, The Netherlands, a.p.buunk@rug.nlAshley D. Hoben, Department of Psychology, University of Groningen, Groningen, The Netherlands)

Something that I find most troubling in this context is the use of the word “cousin.”  It is heroically elastic, ranging from first cousin to radishes – all of course are cousins.  The only living things which are not are direct ancestors and descendants, siblings and aunts and uncles.  So your flu virus is your cousin although your grand uncle is not. 

Biologically this is extremely important.  A study in Iceland (An Association between Kinship and Fertility of Human Couples Agnar Helgason et al. SCIENCE vol. 329 no. 5864 February 8, 2008 page 813 – 816) demonstrated that moderately close kin, roughly third or fourth cousins to oversimplify, had more children than those more or less closely related and that this effect accumulated over generations.  A study in Denmark (Human Fertility Increases with marital radius. Rodrigo Labouriau and António Amorim.  GENETICS volume 178 January 2008 page 603 and Comment on “An Association Between the Kinship and Fertility of Human Couples,” Rodrigo Labouriau and António Amorim SCIENCE vol. 322, page 1634b December 12, 2008) confirmed the phenomenon, which had already been in evidence in all sorts of animal (On the Regulation of Populations of Mammals, Birds, Fish and Insects, Richard M. Sibly, Daniel Barker, Michael C. Denham, Jim Hope and Mark Pagel SCIENCE vol. 309 July 22, 2005 page 609)  I was involved with a study (M.L. Herbert & M.G. Lewis Fluctuation of fertility with number in a real insect population and a
virtual population African Entomology 21(1): 119–125 (2013) that confirmed the phenomenon in a captive insect population and computer model.  There is more evidence.

A study published by the Proceedings of the Royal Society B (Anna Goodman Low fertility increases descendant socioeconomic position but reduces long-term fitness in a modern post-industrial society Proc. R. Soc. B 2012 279, 4342-4351 first published online 29 August 2012 Anna Goodman, Ilona Koupil and David W. Lawson) follows the human experience into as far as great grandchildren and the effect increased dramatically.  Incidentally that last article found that rich people – apparently what you call slow life history – were able to have children who were also affluent, but that advantage did not translate into more offspring down the road; rich people suffer a fertility penalty as long as you care to follow them; presumably they are not marrying cousins as much as the poor are. 

Discussing marriage choice without mentioning the expected number of children and descendants thus is like selling a house without mentioning that it is on fire.  It leaves out something important.  You make a good case for the proposition that cousin marriages are unpopular, particularly among the most productive people.  The birth rate in Mexico is falling and will soon be as grim as for Europe and the rest of the developed world, evidently for the same reason.

Let me know what you think.


M. Linton Herbert MD

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