July 29, 2012
to be posted on nobabies.net
ECONOMIST
111 West 57th Street
NY, NY 10019
Dear Sir:
Anyone should agree that a decrease in teen pregnancies is a good thing. (Setting Aside Childish Things ECONOMIST vol. 404 no. 8795 July 29, 2012 page 26) Among other issues, such pregnancies are harder on women than when they are mature.
You, perhaps accurately, attribute the decline in teen pregnancies in the US (40% from 1990 to 2012 according to the CDC) to increasing prudence on the part of the teenagers. You persuasively dismiss sex education as a factor. The prudence does not stretch to avoiding sexually transmitted diseases – on the rise – nor reduced interest in sex – fewer numbers reporting no sex ever. The question must be asked, “Why now?”
I have been expecting something like this. Unfortunately I have two possible mechanisms. The first relates to a study done 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), which you reviewed. In fact I read your review before I got that issue of SCIENCE. The study demonstrates declining fertility with decreasing kinship of a couple. The error bars are very tight, suggesting that there is not much else dictating the number of children. One thing that does affect that number is the kinship of the grandparents. Excluding some inbreeding depression, the number of grandchildren also falls the same way. The curve does not flatten nor do the error bars grow, indicating that there is an element of post zygotic infertility at work.
The birth rates in rich countries fell and have largely stabilized, but age at first marriage is rising inexorably. Women’s biological clocks are stopping. That would be a post zygotic effect. The timing would hang on the time when people in rich countries stopped marrying their cousins.
The other mechanism observes that an effect being manifest so quickly cannot be due to a genetic mechanism but should be epigenetic, quite possibly having to do with methylation, which is one of the epigenetic processes (occurs during cell division in the germ line, all the time in men but only during prenatal life in females). Several years ago rich countries started adding vitamin B12 and folate to flour for the laudable purpose of reducing neural tube defects in newborns. But these are methylating agents. One sad result, I suspect, is that this has caused bread to taste awful. Few remember how good fresh bread used to be. Another potential effect might be altering fertility by tampering with methylation rates.
Proving either of these would be a monumental effort. However, you may have numbers on your desk that would distinguish between the two. If falling consanguinity is the cause, that should be gradual. If methylation is at the root (and it can only be part of the cause since the decline in teen pregnancies commenced long before girls born under the regimen reached childbearing age) there should be a stepwise decline corresponding to the years since the vitamins were introduced. And that step off should march lock step across the ages of interest. I think it has been long enough so that some regrettably early pregnancies are already occurring in teenagers fed the new diet. The step off should be visible, given good statistics.
It might be a fool’s errand to look into the matter, but since babies are so important, it would be worth looking like a fool and checking it out. Do you have the relevant numbers?
Sincerely,
M. Linton Herbert MD
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