As people and places become richer, women tend to have fewer children. The most obvious reason being that poverty, as in the past, means seeing a quarter of your babies die before their fifth year. So, as wealth increases, this happens less, and it is necessary to have fewer births in order to still ensure that only meaningful result of life, the production of grandchildren.

It is also true that a richer place provides more for people to do other than just pump out more babies, and so, people do more of those other things. In formal economic terms, the opportunity cost (what you must give up to have something) of having children rises as careers and long vacations and all of that open up for women. That glorious economic liberation of women of the past decades, plus a generally richer place, thus means fewer children.

These processes have been happening everywhere, and contrary to certain beliefs, it's not contraception really driving them. Of course, that helps, but the process really started in France long before the invention of reliable artificial birth control methods. We're just in a world where this is what humans do: Richer peoples have fewer children. Contraception definitely aids in achieving it, but it's that desire that has to come first.

The United States was always an outlier in this, though, as there are two processes which militate against this general trend.

One is the influence of religion. Every religion does teach that inculcating the next generation of adherents is a religious thing to do — one doesn't have to be a particularly wise pastor to note that the Shakers are no longer with us. America has long been a more religious nation than us heathens over here in Europe.

The second is immigration. Immigrants tend to bring the fertility patterns of their source with them, taking it into the second and even third generations before they converge on the fertility patterns of the host society. The U.S. has long had very much larger immigration than other rich nations, and since the U.S. has long been the richest large nation by definition, those immigrants are bringing the fertility of poorer places with them.

This is now changing, though: American fertility is down to 1.75 children per woman . Yes, obviously this is an average, but the variations do rather fit our conceptions of religiosity: higher in the Midwest and South, lower on the coasts. There's also been significant influence on that second factor, immigration. Fertility rates have fallen worldwide, so there's less of a different pattern for the new arrivals to bring with them.

Sure, there will be those worried that this means the nation will disappear, that America's not replacing itself. Yet, it's still true that all of this is just what happens. This is what our species does: It has happened everywhere people have climbed out of peasant destitution and left purely Malthusian economic growth behind. It's even possible that we don't like the long-term implication of this even as it happening is proof perfect that pretty much everything else is working just fine.

No rich nation has had, absent those effects from immigration, a birthrate which will replace the population for some decades now. The U.S. was a little later in reaching this point than the others, but that it has done so is just another tale in that story of the decline of American exceptionalism.

We can also look at this the other way around. American women now have the economic and technological freedom to have just as many children as they wish and no more. Whatever the outturn of those decisions, how can we be unhappy with this explosion of liberty and freedom?

Tim Worstall (@worstall) is a contributor to the Washington Examiner's Beltway Confidential blog. He is a senior fellow at the Adam Smith Institute. You can read all his pieces at The Continental Telegraph.