Almost half of all countries have fertility rates below the replacement level, according to an unprecedented study published by the Lancet journal. While there was not a single nation with a fertility rate below the 2.05 threshold back in 1950, the global average is now only 2.4 — down from 4.7 about 70 years ago.

Differences between nations have become more pronounced, with some European countries having record-low rates, down to one child per woman on average, compared to more than six children in some African nations.

If you live in a poorer nation with high fertility rates, a decrease would probably be a reason for celebration. Lower fertility rates are often the result of fewer child deaths, easily accessible contraception and prosperous economies, which explains why Europe, North America and wealthier Asian nations such as Japan are disproportionately affected.

In countries with below-replacement level fertility rates, however, the costs outweigh the benefits. Europe has struggled with this challenge for years. Even more so than the United States, European welfare largely depends on a sufficient number of working-age residents who can finance health care, pensions and social security for everyone. The fewer there are, the more complicated it becomes to sustain a system that was set up in a century when falling fertility rates were among the problems people had to worry least about.

Alarmed by continuously dropping figures, some E.U. governments have taken drastic measures. Italy’s Health Ministry launched an advertisement campaign about two years ago to remind citizens that Sept. 22 was “fertility day.” Other countries have sought to address the issue by focusing on education. Denmark, for instance, has started teaching schoolchildren that having babies doesn’t only come with risks but also with benefits.

But this week’s study raises serious doubts about the impact of such policies or proposals. “Pro-natalist policies have been pursued in more than a dozen countries but the effects on fertility rates have not been large,” they wrote. Instead, they argue, an increasingly large share of the world’s population may have to come to terms with higher retirement ages, slashed benefits and — above all — probably the most divisive issue of this century so far: migration.

Falling fertility rates do not immediately have to result in receding population numbers, as migration and better health care also impact overall figures, according to the study that was funded by the Bill & Melinda Gates Foundation. While almost half of all nations now have fertility rates below the replacement level, population growth declined in only 33 countries from 2010 to 2017. The countries where population figures remained steady despite lower fertility rates also experienced a higher influx of migrants. The fertility rate in the United States is actually also below replacement levels at 1.8, yet the population is still growing, thanks in large part to immigration.

“It takes one generation until the population actually starts to decline,” said Christopher Murray, who worked on the study.

Despite the time delay, there’s another factor that has so far saved the United States from following the fate of Europe’s aging societies: migration. The recent fall in net migration has already slowed U.S. population growth and could eventually accelerate a negative trend.

As other global measures to boost fertility rates have mostly failed, migration has proved to be “effective in sustaining population numbers in several countries,” the authors of the study write, even though they acknowledge that migration has “been accompanied by social and political challenges” in some places.

Based on that analysis, slashing migration numbers — as the Trump administration is attempting to do — and operating a budget that relies on a young workforce appears contradictory. Other countries are increasingly aware of that. Confronted with a record-low fertility rate, Japan decided to relax some of its residency laws this months.

“In the long run, of course, migration can’t be the only fix. Humans would eventually disappear if fertility rates just keep going down,” said Murray, stressing that the United States is still in a better position than many other developed nations where fertility rates dropped below replacement levels decades ago.

“Something that is still up for debate is whether better financial benefits for parents than currently exist might have a sufficient impact,” he said. Northern Europe’s financial benefits are already much more generous than U.S. benefits, however.

With the Trump administration’s recent proposals to slash children’s health insurance programs and to toughen immigration laws, it’s a conclusion that will not sit well among many voters in the United States.

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