In 1957, 4.3 million babies were born in the United States. In 2017, 60 years later, the number was 3,853,472. That’s an 11 percent decline, in a nation whose population has nearly doubled over those six decades. And though there are a few days left in 2018, the number for this year is sure to be lower.

That’s the dominant finding from the thorough — and alarming — report “Declining Fertility in America” by Lyman Stone of the American Enterprise Institute and the Institute for Family Studies.

In recent years, demographic journalists have focused on slivers of the population — the ­increasing percentages of Hispanics and Asians, the decline in births to teenage mothers, low birthrates in high-cost coastal metropolitan areas. Stone looks at the larger picture, that of total population, and finds that “the specter of low fertility, and ultimately of declining population, has come to America.”

That’s a different picture from that of a decade ago. Back then American birthrates hovered around, and sometimes just over, replacement level, roughly 2.1 children per woman. That was a vivid contrast with substantially below-replacement birthrates in most of Europe and Japan.

Those birthrates were buoyed upward by immigrant mothers, after a quarter-century of mass migration from Latin America, especially Mexico. But Mexican migration fell toward zero in the 2007-09 recession, and births to immigrants in the US sharply declined, too.

Some Americans, including many President Trump fans, find that good news. It suggests that a lower percentage of babies are born to mothers in disadvantaged households. And just about everyone, as Stone notes, takes the continuing sharp ­decline in births to teenage mothers as good news, too, considering that such children have tended to suffer negative outcomes.

But the negative outcomes of increasing infertility and eventual population decline have even greater implications. To put it bluntly: Who is going to pay for Social Security and Medicare when there are fewer working-age adults paying taxes for every oldster receiving benefits? Welfare states assume an expanding population, and America’s potential parents don’t seem to be providing one anymore. Why?

Stone rules out one cause: Surveys show that women want more children than they’re having. That was probably not the case, or less so, when American’s fertility rate dropped this low in the middle 1970s.

The culprit this time is something that scarcely existed then: college student loans. The top item on Stone’s list of five causes is “increased young-adult debt-service costs due to student loans.” Number two is “decreasing young adult homeownership” due to higher prices and — here it is again — “student loans.” Number three is “increasing years spent actively enrolled in educational institutions, which tends to reduce birthrates dramatically.”

Government efforts to encourage higher education have backfired for many intended beneficiaries.

Non-graduates still have debt. Graduates with politically correct degrees can’t find jobs. College costs have been inflated by administrative bloat and country-club campuses.

The result is “delayed marriage.” This “changed marital composition explains the vast majority of changes in American fertility over the past 10 or 20 years,” Stone writes.

What are policymakers doing to respond to this abrupt demographic challenge? Approximately nothing. Stone notes that the Congressional Budget Office, the Social Security Administration and Medicare’s actuaries have not “even published stress-test scenarios of long-term fertility at 1.5 or 1.6” — just below the current 1.7.

House Speaker Paul Ryan, the one politician who has worked strenuously to address such problems and at one point got all his Republican colleagues to go along with entitlement reform, has just delivered his farewell speech. House Republicans will be in the minority next month, and with no appetite for taking up the issue again, especially since Trump has promised to leave entitlements entirely in place.

It’s quite a contrast with the late 1990s, when American fertility was higher and Bill Clinton and Newt Gingrich were working on entitlement reform until the Monica Lewinsky scandal broke.

Just another reminder that history is not always a story of progress.