October 29, 2010

Robin Fox
57 US Highway 1
New Brunswick, NJ 08901-8554

Dear Robin Fox:
A good friend has sent me an excerpt from what you have written about marrying cousins.  I plan to post this letter on my blog, nobabies.net, so I make bold to put in an excerpt from the excerpt - reasons for marrying cousins: (Alas I do not have the reference, and if  you could send it I would like to include it in my post.)

Cousin marriage has often been chosen to keep cultural values intact through several generations, ensure the compatibility of spouses, and preserve familial wealth, sometimes via advantages relating to dowry or bride price. Other reasons may include geographic proximity, tradition, strengthening of family ties, maintenance of family structure, a closer relationship between the wife and her in-laws, greater marital stability and durability, ease of prenuptial negotiations, enhanced female autonomy, the desire to avoid hidden health problems and other undesirable traits in a lesser-known spouse, and romantic love. Lower domestic violence and divorce rates have also been claimed.

Good so far as it goes.  But there is a big one missing.  Cousins have more babies.  The cleanest evidence for this is a study done in Iceland.  Here is a graph from that study.

This is what they found:

Sorry I got the name wrong.  The man’s name is Helgason. 

The vertical axis is a measure of the number of children.  The horizontal axis is the kinship, with 2 being second cousins or closer, 3 being third cousins or closer and so forth.  As you see, once you get past 6th cousin fertility drops below replacement. 

I am oversimplifying of course.  A more complete version is attached in the form of a couple lectures I did for a local science fiction club last weekend.  The tone is light hearted, but the science is as good as I know how to make it.  Only the first part will interest you.  Half way though the first lecture I go on with talking about possible life on other planets and the second lecture is about telescopes.

If you want a lot more, you can explore my website nobabies.net. 

I thought you might be interested.


M. Linton Herbert MD

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