Although the great engine of American capitalism is now humming on all cylinders, Americans remain strangely reluctant to have children.

That birthrates plummeted during the Great Recession, which ran from 2008 to 2016, is no surprise. Who would want to bring children into a jobless, stagnant economy? But the return of prosperity after the 2016 election should have produced a quick uptick in births. That, after all, was the consistent pattern we had seen in the past.

Despite generous tax cuts and rising middle-class incomes, however, our birth rate remains mired at the lowest level ever recorded in American history.

How low? At present, we are averaging only 1.7 children per couple, well below the 2.1 needed to maintain our current population.

These dismal numbers presage a sharp downturn in the numbers of American-born citizens in years to come.

Some will hear this and shrug. Our birth rate is still higher than Germany’s, they will say, adding that like Germany we can simply import the workers that we need.

This is like consoling a patient with congestive heart failure by saying that he is better off than the dying patient in the next bed.

After all, Angela Merkel opened Germany’s borders to caravans from the south in 2015-16, partly to show compassion and partly to solve her country’s birth dearth. But what she thought would be a ready-made cadre of workers instead turned into a massive societal and economic debacle. Because of language, cultural and educational barriers, only a minority of recent immigrants to Germany are gainfully employed.

With factories now moving back to the US, and unemployment rates at historic lows, the pressure to import workers into America will only grow.

But it would be far better for the next generation of workers to be “Made in America” than imported from abroad. Most native-born students come to school already equipped — by their own parents —with the language skills and values they need to learn and succeed in America. Many immigrant children don’t and, according to the Federation for American Education Reform (FAIR), they require years of expensive special instruction costing taxpayers nearly $60 billion a year.

Beyond that, Americans should be at the head of the line for the same reason that companies prefer to promote from within — because they understand the company’s culture, speak its language and are more easily accepted by the other employees.

To bolster the birthrate, millennials have to be encouraged to marry and have children.

Right now, because of huge levels of student debt, a preference for cohabitation over marriage and a general unwillingness on the part of young men to take on the responsibilities of marriage and family, they are proving to be a largely barren lot.

Not that I entirely blame them. Millennials have been seduced into taking out a trillion-and-a-half dollars in student loans, which is taking them years, if not decades, to pay back. Who wants to marry someone who is heavily in debt, much less commit to the costly, long-term project of raising children? In this sense, student loans are probably the most effective contraceptives ever invented.

Add to the contraceptive cocktail a hookup culture that does not encourage the kind of committed relationships that are the most fruitful, along with expensive housing and the exactions of government, and you have a recipe for depopulation.

The federal government is not good at instilling values, of course, but it can forgive student loans and reduce the tax burden on young couples.

Forgiving the student loans of those who are willing to enter into a long-term relationship (“marriage”) and have children might prove to be very “fruitful.”

But I would go even further. It seems to me that couples who are willing to raise three or more children should not just have their student loans forgiven, but should be sheltered from all federal taxes.

After all, such couples are providing for the future of their country in the most fundamental way, by providing the future generation, often at great personal sacrifice.

I can already hear the objections of those who will say that such policies will turn young women into unwilling breeders. The truth, however, is exactly the reverse.

There is a surprising amount of what we might call “frustrated fertility” in the United States, especially on the part of young women. According to Gallup, Americans report that their ideal number of children per family is 2.6, a number which has remained unchanged since the late 1970s. This is nearly a full child more than the average of 1.7 children that younger Americans are likely to have if nothing changes.

In other words, forgiving student loans and allowing young couples to keep more of their limited incomes will not in any way limit women’s options. Rather, it will empower many of them by enabling them to act on their deeply held desire to have children.

Another benefit of a higher birth rate is a natural easing of the entitlement crisis. More taxpayers mean more tax revenue. For every one-tenth of an increase in the total fertility rate, for example, Social Security will remain solvent for an additional three years.

John F. Kennedy once said that “Children are the world’s most valuable resource and its best hope for the future.”

Actually, they are our only hope for the future.

Steven W. Mosher is the president of the Population Research Institute and the author of “Bully of Asia: Why China’s Dream is the New Threat to World Order.”