Some East Asian countries have fertility rates near or even below 1.0 per woman, while much of the core population of Europe is shrinking. In the U.S., fertility rates have fallen below replacement rates, hitting a historic low of 1.7 in 2019, and will likely fall even further in 2020 in part due to COVID-19. Many of the world's poorer countries are seeing their birthrates plunge at unprecedented rates. By the year 2100, according to one projection, world population growth will be practically zero.

If you think the world is overpopulated and has serious environmental problems, you might welcome this news. But dwindling populations create their own inexorable logic. If the Japanese population shrinks by half, to 65 million or so, what's to stop it from declining to 30 million? Or 20 million?

There is some evidence that shrinking populations are bad for the global economy. To me, however, the greater tragedy would be a failure to take full advantage of the planet's capacity to sustain human life.

No kind of family policy should be mandatory. But there should be policies that make larger families a more appealing option, both economically and otherwise.

One possibility is that a shrinking population itself will bring self-reversing mechanisms. For instance, a Japanese population half its current size would make Japan an emptier place, presumably lowering land prices. Some families would find it easier to afford a larger apartment in central Tokyo and perhaps decide to have more children.

But that mechanism seems more likely to reduce population decline than to reverse it. Living space is only one of many factors behind decisions about family size. And as population declines, the stock of houses and apartments will decline too, so in the longer run the amount of space per family may not increase by very much.

Population trends depend on how permanent are the causes of fertility decline. In many cases women prefer to pursue careers, or to start having children later, and that means lower birthrates. This same logic would apply in a much less populous Japan or Italy.

Another factor in declining fertility, especially in the U.S., is single parenthood. If a potential mother is facing a fertility decision without another full-time parent on the scene, she is more likely to choose to have fewer children. As population falls, will single-parent families become less common? It is hard to see why. Whether the issue is a lack of marriageable men, unstable family norms or women who simply prefer to go it alone, there is no particular reason to think those factors will disappear in an era of population decline.

If anything, the impetus toward smaller family size might continue or even accelerate. Job opportunities for women may keep improving in quality, which increases the opportunity costs of having a large family. Furthermore, many countries around the world are becoming wealthier. As wealth increases, religiosity tends to decline, and religiosity also tends to boost family size.

What might be some other intervening factors to restore fertility? Perhaps loving robots will make it much easier to raise young children. Or maybe, as populations fall to much lower levels, a sense of moral panic will set in. Families might decide to have more children, feeling that the very survival of their country is at stake. A more elaborate and dystopian scenario would be that corporations take over empty parts of the globe and pay for the raising of children there, in return for a share of their future income.

Undoubtedly there are other unusual (and more utopian) scenarios. Whatever their likelihood, it would not be wise to count on them. It is already the case that in many places, such as Singapore, governments have embarked on aggressive yet ineffective pro-family subsidy policies.

Depopulation is a major problem that the world in general, and its wealthier countries in particular, are failing even to discuss, much less address. In any given year, in any given country, a shrinking population may not be much of an issue, and it may even be welcomed. But make no mistake: Over time, collectively, we are choosing a very different future for humanity.

Tyler Cowen is a professor of economics at George Mason University and writes for the blog Marginal Revolution. His books include "Big Business: A Love Letter to an American Anti-Hero.")