The U.S. population grew by only 7.4 percent over the past decade, the smallest increase since the 1930s, the Census Bureau reported yesterday. This morning, I want to explain why and talk about both the upsides and downsides of slower population growth.

17 percent fewer children than in 1990 — and about 50 percent fewer than in 1960. The U.S. still has a higher fertility rate than Japan and Germany, but it is in the same range as Britain and Sweden and below France and Ireland. There are now more Americans 80 and older than 2 or younger.

The second factor behind the slow population growth is a decline in legal immigration during Donald Trump’s presidency. (Illegal immigration does not appear to have changed significantly.)

relatively flimsy child care programs. Historically, birthrates have declined as societies become more educated and wealthier.

Lower levels of immigration can also have upsides. The big wage gains for American workers during the mid-20th century had many causes, including strong labor unions, rising educational attainment and high tax rates on top incomes. But the tight immigration restrictions of that period also played a role.

“Immigration restriction, by making unskilled labor more scarce, tended to shore up wage rates,” the labor historian Irving Bernstein wrote in a 1960 book. The economists Peter Lindert and Jeffrey Williamson have noted that economic inequality declined more during the mid-20th century in countries with slower labor force growth.

And the big downsides

Over all, though, the slowdown in population growth is probably a net negative for the U.S. — as both conservatives (like Ross Douthat) and liberals (like Michelle Goldberg) have argued.

For one thing, polls show that many Americans want more children than they are having, as The Times’s Claire Cain Miller has noted. But the slow-growing incomes and a shortage of good child care options have led some people to decide that they cannot afford to have as many children as they would like. The decline in the birthrate, in other words, is partly a reflection of American society’s failure to support families.

will be part of his speech to Congress tomorrow night.)

A second problem with slow population growth involves global affairs. The U.S. now faces the most serious challenge to its supremacy since the Cold War — from China. The future path of the two countries’ economic growth will help determine their relative strength. And population growth, in turn, helps determine economic growth, especially in an advanced economy. To have any hope of keeping up with China and its vastly larger population, the U.S. will probably need bigger population increases than it has recently had.

Viewed in these terms, the population slowdown is a threat to national security. “I don’t know of a precedent for a dynamic country that has basically stopped growing,” The Atlantic’s Derek Thompson has written.

In Matthew Yglesias’s recent book “One Billion Americans,” he argues that the U.S. should rapidly increase legal immigration to lift economic output. “America should aspire to be the greatest nation on earth,” Yglesias, the author of a Substack newsletter, writes. The only realistic alternative for that role is China, an authoritarian country that is jailing critics and committing egregious human rights abuses.

Higher levels of immigration also have a direct benefit: More of the millions of people around the world who want to move to the U.S. get the chance to do so.

More from the census report:

• The population continued to shift from the Northeast and the Midwest to the Sun Belt, and California will lose a congressional seat for the first time in history.