April 27, 2013

Faculty of Economics
University of Cambridge
Cambridge CB3 9D

Dear Partha Dasgupta:
I read your paper (Partha S. Dasgupta and Paul R. Ehrlich Pervasive Externalities at the Population, Consumption, and Environment Nexus SCIENCE vol. 340 no. 6130 April 19, 2013 page 324) with the sinking feeling of one trapped in a nightmare and unable to awaken.  You paint a very persuasive case for the notion that on current trajectory human economic activity in its present form cannot be sustained.  One hears (San Charles Our Fertilized World NATIONAL GEOGRAPHIC vol. 334 no. 5 May, 2013 page 95) about nitrogen being used too much in China for instance and too little in Africa, but on the face of it this is misallocation rather than a box canyon. 

Personally a shortage of fresh water where it is needed worries me a bit more.  We were told that greenhouse gasses were going to drive the world’s temperature up over the past ten years, but they didn’t.  So our climate models simply are not the whole truth.  Everyone seems to have heaved a sigh of relief, but my reasoning goes that the climate does change a lot and generally for long periods of time.  What would have happened had we not been adding CO2 to the atmosphere?  Would it have gone colder to the degree that it was expected to get warmer?  Will it get colder now?  Ice ages are dry.  We really don’t need that just now.

But you paint a picture of unsustainability even if nothing unexpected goes wrong.  It wouldn’t seem that more bad news was needed, but alas I have a worry for you that some day you may want to take into account.

One of the strengths of your paper is that you do not take as a matter of course that humans are sort of pleasure maximizing robots.  This is quite in keeping with modern thought, that they are not I mean, (Free Exchange/ the Debt to Pleasure ECONOMIST vol. 407 no. 8833 April 27, 2013 page 72) and indeed one can hardly read the headlines without noticing people doing things that do not add to their own happiness or anybody else’s.  You point out that people are influenced by the people around them, both emulating them and competing with them and occasionally striking out in a new direction.

Along such lines, people can be seen as often economically heedless, wasteful and irrational.  Are we to believe that when it comes to sex and reproductive decisions they are the very model of prudence and rational decision making?  I find that very hard to believe.  And just who is having babies and how many is driving everything else in your nexus. 

In fact, there is a lot of evidence (http://nobabies.net/  see the summary http://nobabies.net/A%20December%20summary.html ) that people do not choose how many babies they want at all.  Their choices and actions and the results are dictated by their own biology.  I have found this to be true of fruit flies and shall attach that paper. 

What is to be hoped is that people actually do have some sort of choice over their mates.  For maximum fertility it appears in humans that the partners need to be kin, say out to third or fourth cousins and their own parents need to be kin.  It is more complicated than that but it is well spelled out in the Helgason study done in Iceland and cited in the link.

It appears very likely that the population trajectory of a community goes like this: you start with a stable population in which the birth rate is just right for the environment.  Then you stir the population up, say by introducing bicycles or cars or anything that permits easier travel and freer socialization.  Then there is a baby boom.  This runs its course.  Then there are two possible scenarios.  If it is still a closed  population, the population will fall and then recover with a second baby boom.  This, too, runs its course.  From there it is the same as with an open population in which the community has assimilated with other communities; the population crashes.

In humans this pattern has only been well documented once, which in the absence of other data would be simply happenstance, but both patterns are repeatedly seen in the mouse plague study, although the depth of collapse there is not so clear as in the Long House Valley data; both studies are reviewed in the summary. 

So what I fear we can expect to see is that the worst news comes first from the developed world, where the birth rate has already completed a cycle and appears to have stabilized.  But it hasn’t.  If you go to gapminder.com
(maybe this link will work: http://www.gapminder.org/world/#;example=75;
you can adjust the graph  thus: go to the vertical axis and under “population” choose the third value from the bottom “age at first marriage for women” and go to the horizontal axis and choose the first value “children per woman.”   Drag the date slider all the way to the left and click “play.”  You will watch birth rates bounce around a bit while age at first marriage stays pretty constant.  New countries join the statistics.  Circles increase in size as countries grow.  Then in 1950 the gates of the bad place open and the devils come out.  One by one the circles move decisively to the left until they are below two children per woman.  Then immediately age at first marriage starts up.  The women are having their children at a later age.  Well we knew that.  But the problem is that once that age starts going up it never looks back.  Fertility never recovers to a viable level, but at least it wobbles a bit.  The rise in age at first marriage never wobbles, never slows and it can’t go on forever.  As that age gets very high, the circle cannot go much to the left: extinction is right at its elbow.  It cannot go up because there is a physiologic limit.  It cannot go to the right because the women are already making heroinic efforts to have those children at an advanced age; they can do no more.  The circle can only come straight back down.  But it never does.  And by the time the statistics run out in 2010 there is not one country that has not started toward the left.  We are all in it together. 

The best news would be that I am just flat wrong and all the data is saying anything but what on the face of it it looks like it is saying.

Getting people to understand this is not simple.  But obviously this is going to have an enormous impact on consumption and the environment.  Some day you are likely to want to figure out what that impact is going to be.  (Again, I certainly hope not, but there are the numbers.)  It won’t all be good news.  The rich, infertile world is a resource for the poor less fertile world.  Certainly much of what the rich world does is destructive.  But it could get a lot worse as populations start dying in earnest.  As for the poor world the question is sort of like a Mark Zuckerberg effect.  It’s nice to get rich before you get old.  I do not know how to predict that but maybe you can. 

Let me know what you think.


M. Linton Herbert MD

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