July 19, 2011
open letter to be posted on nobabies.net

Hania Zlotnik
Population Division
United Nations
New York, USA
zlotnik@un.org

Dear Hania Zlotnik:

I have been taking an interest in population trends and noticed your reply (Population Bomb: the UN responds NATURE vol 474 no. 7353 June 30, 2011 page 579) to Fred Pierce’s discussion (NATURE 473, 125; 2011) of your latest projections.  In some countries estimated fertility is higher than on earlier estimates and in some lower.  The situation is complex but you do not foresee a catastrophic rise in fertility in spite of the fact that the projection for 2050 is 1.7% higher than before.

I acknowledge the complexity of the situation, but I may have a simplifying observation.  Here are some numbers I downloaded from the UN in August of 2008.  They show total fertility in the world broken down into three regions. 
More developed regions:

Less developed regions excluding least developed

 Least developed regions

What I did was simply first to graph them. The result is the graph on the left.

On the right, as the caption describes, I simply cut and pasted.  In the result you cannot even see the edits.  Obviously there is one and only one mechanism that determines the evolution of human fertility in the modern world. 

The mechanism is not hard to find.  The number of children a couple will have, and the number of grandchildren is determined with a high degree of accuracy and precision by their kinship.  This was shown by examining the Iceland genealogy.   Here are the results for children:

Graph is from An Association between Kinship and Fertility in Human Couples, Agnar Helgason, Snaebjoern Palsson, Daniel Abjardson, Pordur Kristjansson and Karl Stefanson, SCIENCE vol. 319 February 8, 2008 page 813.  Notice that “2” in the abscissa really means “first cousin once removed.”  I don’t think it takes a great leap of faith to say that the two curves are similar. 

The implication is very strong.  The world has become richer, social horizons are widening and people world over are marrying on average people with whom they are less and less kin.  Until a significant number start marrying closer than 6th cousin (by their reckoning, which actually was a matter of adding up ancestors 10 generations before and then calculating kinship) we shall enter and remain in permanent demographic decline. 

I told you it was simple.

Superficially both graphs look encouraging.  No dramatic fall is implied and a gentle decline for generations would seem to be ideal. 

However, I have other evidence that I have placed on nobabies.net that suggests the process is not so bland.  In the reductio ad absurdum, it will not take many more generations of single offspring families before almost nobody has kin that close.  More pressingly, when I look at Google’s Gapminder.com and graph total fertility on one axis and age at first marriage for women on another and watch nations evolve over time there is a very consistent pattern.  For most of the past two centuries both numbers bounce about with little evident pattern.  Then each country suddenly experiences a fall in fertility to a level below replacement.  At that moment fertility pretty much stabilizes but age at first marriage starts up.  The timing is so elegant that the system is obviously fine tuned.  It is also evident that there is a pre-zygotic component and a post-zygotic component, which we already knew from the Iceland data (considering both children and grandchildren). 

And obviously things cannot go on this way forever.  In come countries the age at first marriage looks set to reach 40 in a very few decades.  It is hard to imagine stable fertility in such a case.

Please let me know what you think.  If you would like to have guidance through the rather discouraging mass of my website, do let me know.

Sincerely,

M. Linton Herbert MD

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