Birthrates continue to fall all over the world; in Europe they are well below the replacement level. There has been so much population bomb propaganda over the past generation that many people regard this as a good development. Yglesias (tip to Ross Douthat) writes

Less clear to me is why so many people seem concerned by the specter of low birth rates. Historically, low levels of population are associated with high average living standards. That should be less true in the modern world where we’re not as dependent on agriculture for our economic activity. But the logic hasn’t completely vanished. If there were dramatically fewer people in the United States it would be much more realistic for us to all be eating free-range organic grass-fed beef. And even amidst a real estate bust, the country is far too crowded for a middle class family to afford a spacious residence in the most desirable markets such as San Francisco or Manhattan.

There are two inescapable problems with a declining birthrate:

The infrastructure of a city or country is built for a certain population. As the population falls, the infrastructure is too big. In Eastern Germany, sewer systems have stopped function because there is not enough use; rail lines are being abandoned, whole neighborhoods demolished, factories are unneeded, etc.

More importantly, a declining birthrate change s the age structure of a population. The Black Death killed people of all age levels and did not change the age structure of a society; a low birth rate means a rapidly declining AND aging population. A one child birthrate means that hour grandparents are followed by two children and one grandchild. The smaller childhood non-working population is outnumbered by the elderly non-working population. Already in Hungary four million workers support three million pensioners. Whether by private or public means, the retired generation is supported by the working age population, which will shrink and shrink. The cost of supporting retirees will further discourage the working-age population for having children, and the downward spiral reinforces itself.

These are the inescapable problems. There can be other problems depending on historical circumstances. When there are differential birthrates in a society, when one group is out-reproducing the other, severe problems can result. Lebanon was torn apart by war when the Muslim population became the majority because the Christian population, influenced by the French, had smaller families. Israel faces a majority-Arab population because the Arabs have more children than Jews do; and in many European cities Moslem children area already the majority of the school-age population. The Serbs lost Kosovo to the ethnic Albanians because the Albanians had children and the Serbs did not; ethnic Slovakians face a near future in which Gypsies may be the majority, for the same reason.