September 27, 2011
To be posted on

Laura Blue
The Office of Population Research
Princeton University
Wallace Hall
Princeton NJ  08544

Dear Laura Blue:
Your article on immigrant health (Laura Blue The Ethnic Health Advantage SCIENTIFIC AMERICAN vol. 305 no.4 October 30, 2011 page 30) was an eye opener for me, more than you can imagine.  The gist is that immigrants generally and Hispanic Americans particularly live longer and healthier lives than people born in the host country despite a relatively lower economic status, which ought to produce the opposite effect.  And you find most of the difference to be due to the fact that immigrants tend to smoke less.  Smoking kills.  I remember the term “coffin nail” being used for a cigarette long before there was any official announcement that a cigarette was deadly. 

The thing I find mind numbing is that word “most.”  Even taking smoking into account the paradox of healthy immigrants remains. 

I would not have believed it.  My own interest is in fertility.  It turns out that a moderate degree of kinship – third cousin is probably ideal, or maybe fourth, don’t even think about going out to eighth cousin – is necessary for normal fertility both in the first and in the second generation. 

Until the past few days I had taken it as axiomatic that this boost in fertility is a trade off with a loss of overall health.  I think there was an article “Longevity and the Barren Aristocrat” in NATURE some years ago.  I thought it was too obvious to keep track of.  Those women in the extensive British genealogy of the gentry who had few children tended to live longer.  So I thought the fertile ones – necessarily inbred, since that is the strongest determinant of fertility (A. Helgason et al., Science 319 813 (2008) – were dying young through a combination of stress and inbreeding. 

But my abject acceptance of common superstition that inbreeding is a Bad Thing is now challenged.  I have no numbers, but I am quite sure that the immigrants and Hispanics you have taken an interest in are moderately inbred.  Hence they have an adequate number of children while the native born non-Hispanics do not. 

Inbreeding should have made them less healthy overall.  And now you have very credible statistics showing that they are in fact more healthy.  That makes a lot of sense.  If you look at the article I mentioned you will see how rapidly infertility develops; it only takes a few generations.  Evidently evolution is not happy with us trying to maintain large gene pools, more than a couple hundred; certainly a thousand would be too many. 

I learned recently that the overall incidence of full blown mental illness in infertile (and thus evidently seriously outbred) Europe is 38% (H. U. Wittchen et al. Eur. Neuropsychopharmacol. 21, 655–679; 2011) and in critically infertile (and thus terminally outbred) Eastern Europe it is much worse.  That may come as a surprise, but it should not be mystifying.  If something can destroy fertility, which is the top priority for evolution, it certainly could destroy mental health.

Since infertility develops so rapidly it cannot be due to DNA mutations.  They are far too rare.  It must be due to an “epigenetic” effect, having to do with the control of genes.  Recent work has shown that inbreeding depression, at least in one species of garden flower, is not genetic but epigenetic.  (A review in SCIENCE (Elizabeth Pennisi Epigenetics Linked to Inbreeding Depression SCIENCE vol. 333 no. 6049 September 16, 2011 page 1563 of work done by Philippine Vergeer).  Inbreeding depression is removed when epigenetic effects are removed. 

So there seems to be a single set of mechanisms that reduce fertility, mental health and overall health of offspring of couples that are too closely kin and too distantly kin. 

That is how it looks at this point.  I confess my head is still spinning from this, but evidence is evidence.  Perhaps your statistical magic can sort this out.  We would be much better off if we knew the truth.


M. Linton Herbert MD 

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