May 28, 2012
To be posted on and

Professor Patrick Bateson

Dear Professor:
An internet friend known to me only as “sNoOOPy” (do you suppose he is a spy?) pointed out your book Mate Choice (Cambridge University Press Cambridge 1983) and I have read it with great interest.  Your chapter Optimal Outbreeding suggests that for mating choice there is an optimal degree of relatedness.  You demonstrate this elegantly with an experiment on Japanese quail, which are shown to prefer first cousins, but you conclude that evidence is “meager.” 

There may be little evidence with regards to choosing an optimally outbred mate, but there is an enormous amount of evidence that it is a jolly good strategy.  One paper (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) assembled almost two thousand serial field counts or different kinds of animals and showed that population growth rate was a function of population size in a fashion that could not be accounted for by the environment.  Briefly put, the cost of outbreeding is a failure of fertility to below replacement levels on average.  So your concept is totally vindicated in terms of their being a need for optimal outbreeding.

The same thing has been proven in humans (An Association between Kinship and Fertility of Human Couples Agnar Helgason et al. SCIENCE vol. 329 no. 5864 February 8, 2008 page 813 – 816 and a couple papers from Denmark).  The authors discounted social effects like limiting family size so as to be able to pay for college, which at all events is not found among other animals.  Interesting to me is that they found that optimal outbreeding in Icelanders means choosing a second cousin or closer.  (They calculated kinship by going back ten generations and counting shared ancestors, not the usual shortest-route method.)  Your Japanese quail chose first cousins, which is quite consistent.  Among the Vikings there, it was a slightly different story in the second generation.  Optimal outbreeding was “third cousin or closer to fourth cousin or closer.”  That to me sounds like second cousin once removed to third cousin one removed with the optimum being third cousin, not around first cousin.  I think one might suggest that a typical bird egg has a less than even chance of producing another bird egg but even in these late and dwindling times a human baby has a better than even chance of producing another baby.  My poor brain struggles with the abstraction, but my feeling is that birds would do better to consider the first generation while humans should consider the second. 

My life has developed into a campaign of telling people who ought to know that you can have too much inbreeding or too little.  I get blank looks from just about everywhere.  Exalted experts fail to return messages.  Your observation that people think that inbreeding is “bad” is as valid as ever or more so.

I sort of thought I was onto something new.  You were many years ahead of me and you are absolutely right.  Indeed you give a reference to a similar notion that goes back to 1933.  It’s only common sense.  You can have too much or too little of anything.  I guess my own contribution is, “This is very important.”  It gives us for the first time a rational basis for mating choice in humans.  That would be trivial if our current quite irrational approach were working, only it isn’t.  There are too many people.  At the same time there are many countries with a serious lack of babies.  Do you suppose maybe we ought to grow up and face reality?

Let me know how you feel about this.  If you have anything to suggest to the readers of my blog, do let me know.  And if you have any idea as to how to proceed, I would be frantic to know it.

By the way, I have a research paper relevant to the topic – fertility –  that is, not exactly choice.  (I don’t mean the paper isn’t choice, it’s about choice.) Any ideas for a referee?


M. Linton Herbert MD

Note to the web site:
For the record, Professor Bateson has sent a prompt, warm, courteous and very helpful reply.  (There was a delay between my writing the letter and sending it to him.)  I shall extend my gratitude to him as soon as I have studied material he sent.
Thank you, Professor.

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