We’ve written about demographics a number of times over the years, expressing serious concerns about the U.S. population and that of the world at large. University of Tennessee law professor Glenn Reynolds shares those concerns, writing:

I was born in the 1960s, just about the time people decided it was bad for children to be born.

Oh, I don’t take it personally. It’s just people started to worry about a “population explosion.”

Thanks to antibiotics, vaccines, mosquito eradication and better farming, people weren’t dying off as they had been.

But they were still having babies.

This led to more people.

And quite a few folks — themselves already born — thought that was bad.

The most famous is Stanford’s Paul Ehrlich, an entomologist whose bestselling book about people, “The Population Bomb,” promised the Earth a grim Malthusian fate, only a decade or so away.

We’d see mass starvation, he predicted, and food riots in American cities before the 1970s were out.

Not only did those predictions fail to come to pass, but one of the biggest problems today is obesity. Yet policy makers listened to Ehrlich anyway, and the results have been and will become even more devastating.

Fewer people are being born; in most countries outside Africa, nations (including the United States) are not producing enough people to replace the ones dying.

This means the population will shrink, and the average age will go up.

Some people think that’s fine.

The world was doing OK with 3 or 4 billion people before, so why should we worry having that few people again?

The trouble is 4 billion people on the way up is a very different population from 4 billion on the way down.

The former was young and dynamic, with productivity increasing and risk-taking popular.

The latter will be older, probably with a lower appetite for risk, and becoming less productive as it ages further.

While some nations try to encourage making more babies, others — like our own — are simply relying on mass immigration. However, warns Reynolds, “If your native population is shrinking as immigrants pour in, it starts to look less like reinforcements and more like replacement.”

He asserts and ponders:

Children are a blessing but also, especially in the early years, a sacrifice.

I sometimes wonder whether the sexual revolution, which stressed nonreproductive sex, flourished in part because the Ehrlich message made that sound virtuous rather than selfish.

He concludes with this insight:

By the turn of the next century, we may see a world dominated by the descendants of Amish, Orthodox Jews, evangelical Christians, traditional-rite Catholics and fundamentalist Muslims.

Read the whole thing here.