It's teh retiree ratio, yo

Matt Yglesias wonders why low birth rates are a bad thing:

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.

If only population could be changed without people growing old and dying! But unless we are prepared to eat the old (Soylent Green, anyone?), shrinking without aging is unrealistic. An aging population means fewer workers per retirees - more pressure on pensions and health care costs. That puts a drag on the economy, holding down living standards and increasing political unrest.

Poor, overcrowded countries like Bangladesh may have to make a choice between getting rich and keeping a stable population structure; there's just too many people for the land to support. But rich countries like the U.S. and Japan have already shown that we can live within our means with our current populations; for us, the ideal fertility rate is the replacement rate, 2.1 children per woman.

Which, if you look it up, turns out to be exactly the American fertility rate. Finally, there's something we don't have to worry about.