Mothers and fathers die. For the population to remain constant, the two of them need to produce about 2.1 children on average in order to replace themselves. (The extra 0.1 is necessary to offset the deaths of children who never reach reproductive age.) This figure is called the “fertility rate.”

Recent data shows that the fertility rate in the United States is nowhere near that 2.1 figure. It has instead dropped to an all-time low of about 1.6. American woman on average give birth to only 1.6 children.

This raises a question: How fast will our population decline due to our low fertility rate of 1.6 compared to the replacement rate of 2.1?

The answer is, pretty fast. At our current fertility rate of 1.6, the U.S. population will decline by 50% in 77 years.

Granted, that calculation doesn’t account for immigration into the country. But are we willing to accept an enormous number of immigrants – something like 200,000,000 – over the course of three-quarters of a century?

That would constitute the biggest immigration wave in the history of the world. It would be much bigger, on both an absolute basis and percentage basis, than the massive waves of the Irish to America in the 1800s, Italians in the 1900s, and even Latinos in the 2000s. Immigration on that scale is inconsistent with the current political sentiment and, arguably, inconsistent with maintaining our American culture.

If it makes you feel better, know that the fertility rate in most of Europe is even lower. In Italy, it’s about 1.2. At that fertility rate, the population of Italy will decline by about 75% while the population of America declines by “only” 50% over those 77 years.

Extend that out further. At a fertility rate of 1.2 over 800 years, the population of Italy will decline to approximately 20 people. At least there won’t be any complaints about a housing shortage.

(BTW, consider the fact that this ridiculously low fertility rate in Italy is in a country that is nearly universally Catholic, and that the Catholic Church prohibits abortion and contraception. Along with art and food, the rhythm method has apparently been perfected in Italy.)

Back to America. The bottom line is America is apt to shrink. I see some good and some bad in that.

First the good. As a matter of personal aesthetics, I think we have enough people here already. The roads seem sufficiently crowded. Even hiking in the remote mountains of Colorado, it’s rare that I wish there were more people on the trail.

It’s true that our cities are hollowed out, but that’s due to crime, corruption and mismanagement, not a lack of people.

Now the bad. Our economy, like most modern economies, hinges on growth. Imagine an economy where the gross domestic product is flat forever. Imagine an economy where the stock market doesn’t go up and everyone’s 401K stagnates.

Worst of all, imagine an economy where the number of young workers paying Social Security taxes drops dramatically due to low birth rates while the number of retirees collecting benefits rises dramatically due to longer life spans.

This touches on a basic issue. Our Social Security system is a big pyramid scheme. Old people like me tell ourselves that we’re simply collecting the money we paid in over a lifetime of work, but, in point of fact, the average retiree collects far more than he paid in. That’s possible only because the number of payors continues to grow faster than the number of payees.

At our current fertility rate, expect a collapse sometime in the next few decades. We’ll lack the workers to fill the bottom of the pyramid.

Finally, there are some aspects of our de-population explosion that are philosophical, metaphysical and even religious.

God told the Abrahamic religions to “be fruitful and multiply.” We obeyed. Did we ever.

This advice to “be fruitful and multiply” was given twice. The first time was at Creation when the human population was two people. The second time was after the Flood when the population was eight people.

We haven’t heard that advice for a long time. During that long time, we’ve multiplied and fruitified to a population of eight billion people. If the advice is still applicable, then when will it expire? When we’re eight trillion? Eight quadrillion? Do we just keep multiplying and fruitifying until there’s no place to stand?

In the philosophical realm, there’s this. We seem to be the only form of life in the universe that is capable of asking questions like “How many of us are enough?” And, from the evidence found so far, we inhabit the only home of all life. As a philosophical, moral and ethical matter, what is our obligation to propagate? To what extent? How many? Where?

These questions are not easy, but it’s worth talking about them. We might be the only creatures anywhere at any time to have that talk.

Note to readers: In case you’re wondering, I fathered at least two children, and one of them gave birth to my grandchild a few months ago. Two of the three pay Social Security taxes.