Since 2005, per capita gross domestic product has grown on average by 0.6 percent a year in the United States, exactly the same rate as in Japan and virtually the same rate as in the 19 nations of the eurozone. In other words, if it weren’t for the boost from babies and immigrants, the United States economy would look much like those supposed laggards, Europe and Japan.

Indeed, if the United States population had been growing as slowly as Japan’s over the last two decades, its share of the global economy would be just 15 percent, not the 25 percent it holds today.

Moreover, immigrants make a surprisingly big contribution to population growth. In the United States, immigrants have accounted for a third to nearly a half of population growth for decades. In other countries with Anglo-Saxon roots — Canada, Australia and Britain — immigrants have accounted for more than half of population growth over the past decade. Those economies have also been growing faster than their counterparts in the rest of Europe or Japan. But much of that advantage would have disappeared without their population advantage.

Politically, the irony of this moment is stark. Population growth is increasingly important as an economic force and is increasingly driven by immigration. Yet now along comes a new breed of nationalists, rising on the strength of their promises to limit immigration. And they have been especially successful in countries where anti-immigrant sentiment has run strong, including the United States and Britain.

Immigration has been expanding the United States population at a rate of about one million people a year over the last decade. It’s not clear exactly how much President Trump’s policies will reduce the net flow of immigrants, given that he has yet to articulate a broad-based immigration program. Prime Minister Theresa May of Britain has been more explicit, promising to cut the net inflow to the “sustainable level” of under 100,000 immigrants a year, down from an average of 250,000 in the last decade. For these leaders, delivering on these anti-immigrant messages may now be politically imperative, but it will seriously handicap their economies in the global growth sweepstakes.

History is littered with examples of emerging nations that have failed to generate enough jobs for a booming young population. But virtually no nation has ever sustained rapid economic growth without strong population growth. And at a time when every major country including the United States faces continued decline in population growth, workers are an increasingly precious source of national economic strength.

In the long run, governments have limited avenues to increase the growth rate of the labor force, which is unaffected by short-term fluctuations in unemployment. Even enticing the “forgotten men” — those no longer looking for work — back into the labor force can have only limited impact. The main reason fewer Americans participate in the labor force is not because they are discouraged, but because they are getting older.