May 20, 2015

Martin Rees
Center for the Study of Existential Risk
Centre for Research in the Arts, Social Science and Humanities
Alison Richard Building
7 West Road
Cambridge CB39DT

Dear Martin Rees:
First accept my thanks for being out there assessing new risks to the survival of humanity.  But there is an old risk that is much neglected.  Briefly: if a population of humans becomes urbanized, get too large, it will die out.  It is well established that when a population becomes prosperous that the birth rate will fall.  It will stabilize for a time generally between 1.5 and 2 babies per woman.  At that rate the population would decline serenely over centuries until we were below the 2,000 the world can support so all seems well.

However there is a study (attached) by a man named Calhoun, who put some mice in a large cage taking great care to have enough resources to avoid overcrowding.  The population grew, growth slowed and then on a day live births ceased never to return.  He attributed this to a psychological problem and on the basis of that predicted societal collapse around 1992.  The year has come and gone, leaving us with no explanation except a genetic one: too much diversity kills you off.  Jarred Diamond published a brilliant study (reference at
in which he followed the time course of Anasazi Indians in long house valley in the American southwest.  This was before any European contact.  People can be seen moving in as there are stepwise increases in the population but never any clear cut stepwise decrease.  They died out.  He blamed it on climate since tree ring growth paralleled the population, but I believe they were cultivating trees.  The time from being a small village size population to extinction was about 300 years.  There was a notch about mid trajectory.  The survival rate of Japanese and Chinese dynasties follow the same trajectory (same reference) including notch at the same time and 300 year maximum survival.  A review of Mesopotamian civilizations (same reference) shows the same 300 year brick wall.  With steady decline as that age is approached. 

I surmise that the effect is epigenetic and published a paper in African Entomology (attached) in which I was able to show roughly the same pattern in a computer generated population affected both by pre-zygotic and post-zygotic effects.  So the effect seems above question.  The only question is whether there is time to fix it.

Since different contemporary societies are presumably on different places on the trajectory, but there is a fraction (called by Gregory Clark in The Son Also Rises those with general social competence) that is probably vital to the function of any society.  That would include you but likely not me.  You all are, so far as I can tell, probably very nearly on the same point on the curve.  Absent you, our high tech civilization vanishes and you are left with 10 billion people with a technology able to support 2 billion (that’s American billions of course) under the bests of circumstance, and circumstances will not be the best.  Social upheavals will likely scatter whatever tight, and thus long tem viable, bands remain.  The curtain falls.

One of the icons of life is the 14 year girl hearing her parents say, “It’s going to be all right.  There are lots of other fine boys in the world.”  She tries to explain through a cataract of tears, “But you don’t understand.  He has to love me.  The world has gone mad.” 

She likely may have chosen the wrong boy (of course it would have to be a fairly close cousin, like third through seventh) but she is right in the importance.  So how old and how big is this?  Permit me to hype a bit.  The attachments and reference cover the important stuff.  

A few years back a man found a rock on a beach.  He had been looking for one like it.  It was a stone axe made, he could tell, by a Homo erectus.  Since it was the second such axe found on the island, and since the island had been reachable only by open ocean travel ever since before any kind of human, and since the axe had been left there a million years ago, it seems clear that seafaring was practice that long ago.  Yet the time from Columbus to us walking on the moon was less than 500 years.  Demographic collapse never let us get this far before.  (Why we should have managed of course must be due to mating patterns in England, where the Industrial Revolution started.  Those patterns have been abandoned.) 

And how big?  Somebody has figured that our society should be able to colonize the galaxy within 50 million years.  Since the galaxy has changed little in the last few billion years, any civilization as old as ours should have long since been here, and its presence would be obvious.  If getting as far as we have were easy, the chance that all the others were less than 50 million years old is miniscule.  We are alone in the galaxy.

There are a lot of other galaxies, but ours is very unusual.  We are big and old, so there are lots of elements greater than lithium around.  I doubt there is much chance of life forming out of only hydrogen, helium and lithium.  But big galaxies have active nuclei, which would destroy any life.  That is what is different about us.  Our nucleus is quiet.  The rare galaxy out there just like ours is rare and, unless reaching our state of development is easy – and we know it isn’t – we are alone in the universe.  Absent us the universe is insane. 

All those dewy eyed maidens have been right.

Of course I know you are most interested in new things, but the name of your foundation implies that the threat may not be new.  I earnestly hope you or perhaps Huw Price find it in your heart to consider confronting an ancient horror.


M. Linton Herbert MD

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