Back to the stats:
I just wrote a letter to Partha Dasgupta concerning her paper about population, consumption and the environment.  I thought she might be interested in where it looks like the population is going.  To illustrate my point I directed her to the web site where there is a wealth of statistics for most countries put together in a convenient form by Google.  I find some of the writing hard to read on my monitor so I tried to be explicit in where to find the data I think is interesting. 

As I point out in the letter, for any given country the fertility bounces around happily for decades.  Then it starts to fall abruptly, leveling off below replacement.  Then it more or less stabilizes; some countries have a rise in fertility but never past replacement.  But as the birth rate stops falling the age at first marriage for women starts up.  And it never stops climbing. 

In fact I have a dear friend who is now pregnant with her first child.  She is about forty.  It would be rude to ask, “What took you so long?”  But it is an important question from a theoretical standpoint.

The paper I have referred to in which I was involved explains that it appears to be true that the infertility that comes from outbreeding has at least two components.  One is a post-zygotic component.  (Pre-zygotic infertility means that the sperm cannot fertilize the ovum; post-zygotic infertility means that the fertilized ovum does not develop into a fully fertile adult.)  That seems pretty straightforward.  Sperm counts are falling so consistently that the UN keeps redefining what a “normal” sperm count is downward.  Male sexual development is less pronounced and more often goes awry.  Well and good.  That has to be a post-zygotic change.  There may be other post-zygotic components to the mechanism of course.

And there must be a pre-zygotic component.  That one is probably less intense, takes longer to develop and saturates out so that in isolation it cannot produce absolute infertility within a species or even between closely related species.  In the summary I refer to in the letter I mention that it has been shown that sperm can recognize kin and bind to them in the dear mouse species.  If sperm can recognize kindred ova and bind to them better, that would be a pre-zygotic component.  That’s not known to be true at this time.

So the fall in fertility we see, since it develops first, ought to be post-zygotic.  But what about the rise in age at first marriage for women?  Is it pre-zygotic or post?

If this is due to women’s biological clocks slowing to a stop, then that ought to be post-zygotic.  If it is because they are looking for somebody they love enough to marry – somebody who would be a cousin – and not finding him, that would be pre-zygotic.  And the timing would be right.  In the statistics package the two components appear to be elegantly tuned to each other.  I tend toward the latter interpretation, but I am not sure.  That means there would be two components to the pre-zygotic component. 

I don’t think it’s important from a practical point of view.  If people don’t care, the mechanism doesn’t matter.  If they do care then correcting things doesn’t depend on the mechanism.  It depends on mating strategy.

But considering what to expect, there it may matter.  If the rise in age at first marriage – or first child if you prefer – is post-zygotic, then that rise should continue to power upward until the birth rate falls effectively to zero.  If it is pre-zygotic, then it should stop because the pre-zygotic component saturates.  In that case, we might have a little more time before the real pre-zygotic component cuts in.  So hope that it’s love and not biological clocks.  If anybody has an opinion to volunteer so I don’t have to be rude, it would be much appreciated. 

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