Is the descent linear?
Anyone who cares to check the numbers will see that the world birth rate is falling.  It is a trifling matter to compare the fall in different sections of the world and notice that the descent appears to follow a single curve everywhere, implying if we knew nothing else, that the descent has a single cause everywhere.  If you can think of anything except a genetic cause that could be so universal, or if you can think of any genetic issue that is universal other than the increasing mobility of people and increasing pool of mates to choose from, then you are far cleverer than I.

But we have called that line of descent of birth rate linear some times and curved other times and crowed that we now could understand why.  So it is a valid question as to whether the curve is linear or not.  The obvious answer is that it depends on the scale you choose.  Looking at the world birth rates over a fifty year interval, the curve looks anything but linear.  Looking at German birth rates as reflected in the age distribution, the line looks fairly straight.

And in trying to guess how rapidly the crisis is coming upon us, it looks like the scale is closer to fifty years than to five in terms of birth rate.  But if you do the linear extrapolation, the time to no children destined to be fertile looks like it is on a scale of five years.  The quandary is great.

Although we have any number of societies that have faced the crisis before, none seem to have left us with good census data.  The Romans evidently tried.  You remember the Christmas story beginning, “And there went out a decree from Caesar Augustus that all the world should be taxed.”  And everybody had to go to their home towns to be “taxed.”  That would be odd.  You generally tax people where they work, or at least on the basis of the fact that they work; you do a census at home.  I understand the word “taxed” has been rendered “registered,” in some translation, which would make better sense.  But the records of birth rates to my knowledge have vanished; I mean the actual counts. 

After the Romans, the British would have been the best bet.  They did keep careful records of births at least among the landed gentry, but the British have defied the odds.  Evidently they all unwittingly devised a mating strategy that let them elude the specter that so haunts history elsewhere.  So their records do not tell us what to expect but rather what not to expect. 

We have seen in multiple places, with the computer model, with world birth rates and with the experience with black footed ferrets, that the birth rate fall does seem to level out.  In fact, if you look at the last interval in the birth rate of the developed part of the world, there is even a slight upturn.

Mathematically, we would expect that fertility should find a bottom.  When the numbers become sufficiently small, the birth rate should rise again to about what it takes to survive.  But that depends on the numbers being very small indeed.  It is hard to believe that the developed world has thus found bottom and the birth rate will rise to replacement levels if nothing is done. 

During exponential growth, the computer model, the world birth rate and the black footed ferret populations are in exquisite agreement, showing a modest fertility boost while the populations escape inbreeding depression and this is verified by the Danish study.  We know how it begins.  But we don’t know exactly how it ends.  Just when we need the best data, we have almost no actual numbers from the past, only overall trends.  The most minimal prudence dictates that we must assume the worst.  But I cannot actually prove that the worst will be the event.  I have, however, proved that things look very bad indeed.

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