Ana M. Viamonte Ros, M.D., M.P.H.
Surgeon General of the State of Florida
4052 Bald Cypress Way
Tallahassee , FL
Phone: 850 -245-4147  
Fax: 850 - 487-4574

Dear Dr. Ros:

I am writing about a matter of great importance with regards to the health of the people of the state and elsewhere.  As is well known, the fertility of people is declining rapidly on average all over the world.  What is less well known is that there is a strong tendency for fertility to level off at somewhere between 1 and 1.5 children per woman.  Given that we are on a finite planet that we are already overtaxing, this all adds up to a comforting picture.

However people continue to want children and often have difficulty with that.  What is hardly known at all is that kinship is related to fertility.  Yet facts have been published in prestigious scientific journals.  Let me list the four that I think are the most relevant.

  1. On the Regulation of Populations of Mammals, Birds, Fish, and Insects, Richard M. Sibly et al. SCIENCE vol. 309 no. 5734  JULY 22, 2005 page 607 – 610
  2. An Association between Kinship and Fertility of Human Couples, Agnar Helgason et al. SCIENCE vol. 329 no. 5864 February 8, 2008 page 813 – 816
  3. Human Fertility Increases with Marital Radius, Rodrigo Labouriau et al. GENETICS vol. 178 no. 1 January 2008 page 601 – 603
  4. Comment on “An Association between Kinship and Fertility of Human Couples,” Rodrigo Labouriau et al. SCIENCE vol. 322 no. 5908 12 December 2008 page 1634 – 1635


The first article describes an analysis of more than 1,000 field studies of animals.  They did not find inbreeding depression in the wild.  What they did find was that in small populations growth rate – and hence fertility – actually fell rapidly as population size increased and then leveled off below replacement level.  This surprising result was seen to be unrelated to environmental factors. 

The second article looks directly at the kinship of people and the same curve is found.  Past second cousins, the number of children and the number of grandchildren fell rapidly with decreasing kinship and then leveled off below replacement. 

The third and fourth articles taken together show the same relationship based on inferred kinship based on distance between birthplace in Danish couples. 

Given three independent papers, one summarizing over a thousand studies, and all pointing to the same effect it appears that science has spoken.  Those who want babies must marry kin or face a significant risk of disappointment or disappointed children. 

In medicine we have means to encourage fertility, but these means address specific couples.  For reproductive health people need to know at an early age what the science is so as to be able to take it into account during courtship.  It is possible to argue that it should be taught in public schools since it is a matter of public interest.  That is a matter of judgment of course. 

What is more pressing is the fact that almost nobody knows what the science is.  No official statement has been made, such as was made about cigarettes and health, warning people of the consequences of their choices. 

I hope you will look into this and make an appropriate assessment so that the rest of us doctors understand. 

I have spent the last several years watching this evolve and looking into the historical effects of birth rates, which is rather easier to do than demonstrate directly the relationship between birth rate and fertility.  I am ready to do anything in my power to help you if you decide to move forward.

I am sure you will want to call in experts and quiz them.  I should caution you that the Sibley study was done in England, the Helgason study done in Iceland and the Labouriau studies done in Denmark, so calling them in for a chat would not be easy.  As regards consulting other experts, my experience is that the prejudice against kin marriages is as strong – and alas as irrational – among professionals as among others, so you may find yourself having to insist on the facts being discussed rather than accepting a comfortable generalization based on commonly held belief.

Perhaps it would be best to convene a committee to look into the facts and come up with a prudent analysis.  As I said I am at your disposal.


M. Linton Herbert MD 

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