Many a young women has looked inside herself, decided that she just doesn’t want kids, and went on to live her life under that assumption.  But a decade or so later, her biological clock suddenly went off and she found herself very much wanting kids.  I’ve seen this happen several times.  None of us should be very confident about what introspection tells us we will later want.  Evolution has designed us to express different genes at different ages; we just can’t know what future genes we have been designed to express.

This caution should apply all the more to humanity and its descendants as a whole; we just can’t look inside ourselves to discern our future fertility.  Since the industrial revolution made us rich, we’ve seen the remarkable phenomena of the demographic transition, whereby after nations get rich population growth rates eventually fall dramatically, often below replacement levels.  This is not the sort of adaptive behavior that would have evolved had our ancestors repeatedly encountered such extended boom times; clearly we could now leave more descendants by using this opportunity to breed like crazy.

Our evolved instincts must be framing our novel situation as similar to some ancient situations where having fewer kids actually did make sense.  We still don’t understand this well, but we can see that those evolved instincts are mistaken.  At the moment contrarian subgroups who choose more kids (e.g. Hutterites, Hmongs, or Mormons) are slowly increasing as fraction of the population.  The more heritable, either genetically or culturally, is this fertility behavior, the more these groups will come to dominate future global population and fertility.

Of course we can’t say much with confidence about any one group; they may well fragment or become assimilated by larger groups.  But what we can say with more confidence is that if our society continues to be competitive, without strong central coordination, selection will be a powerful force influencing fertility over the long run, e.g. a thousand to a million years.

Yes it can take a long time for selection to favor behavior that starts out in only a small minority, and yes our descendants may have very different bodies whose designs are encoded not in DNA but in computer files.  And yes, for a time individual regions or nations may forcibly limit the fertility allowed there.

But as long as enough people are free to choose their fertility, at near enough to the real cost of fertility, with anything near the current range of genes, cultures, and other heritable influences on fertility, then in the long run we should expect to see a substantial fraction of population with an heritable inclination to double their population at least every century.  So if overall economic growth doubles less than every century, as I’ve argued it simply must in the long run, income per capital must fall over the long run, a fall whose only fundamental limit is subsistence; we can’t have kids if we can’t afford them.

Added 8p: Does anyone doubt that the main reason humans stayed near subsistence level for a million years until about 1800 was selection for behavior giving maximum sustainable population levels?   What exactly is going to be so different over the next million years?

Added 24Sept: Bryan responds here.