Fertility in America has been declining for years. According to the Pew Research Center, the nation’s birth rate hit an all-time low in 2011 – just 63 births per 1,000 women of childbearing age. It was almost twice as high – 123 births per 1,000 women – at the peak of the Baby Boom in 1957.

As babies and children disappear from a society, what takes their place? One answer, as journalist Jonathan V. Last observes in a forthcoming book, “What to Expect When No One’s Expecting,” is pets.

In surveys taken from the 1940s to the 1980s, fewer than half of Americans said they owned a pet. Today America’s 300 million humans own 360 million pets. Last puts that in perspective: “American pets now outnumber American children by more than four to one.” Often those pets are pampered to a degree that quite recently would have been thought eccentric. The average dog-owning household’s spending on pet grooming aids, for example, more than doubled between 1998 and 2006. Last notes that when a kids’ clothing store in the suburban Washington neighborhood where he used to live went out of business, it was replaced by a doggie spa – leaving the neighborhood “with six luxury pet stores and only two shops dedicated to clothing children.”

A mania for pets isn’t all that materializes when the birth rate sinks. So do economic stagnation, dwindling innovation, a declining lifestyle, the exploding health and pension costs of an aging population, and the ever-heavier taxes needed to maintain the government safety net when there are fewer workers and entrepreneurs. Optimism, booming markets, and technological dynamism recede, supplanted by intergenerational conflict and loneliness.

Many people, it’s true, are still in the grip of the Malthusian fallacy. The superstition that that the Earth is already too full, and that more human beings will mean more hunger, misery, and environmental despoliation, is a popular one. But serious demographers, economists, and others have been warning for years that declining populations lead to shortages, misery, and upheaval.

“If you think that population decline is going to be a net boon to society,” Megan McArdle writes in the Daily Beast, “take a long hard look at Greece. That’s what a country looks like when it becomes inevitable that the future will be poorer than the past: social breakdown, political breakdown, economic catastrophe.”

If so, Greece will have plenty of company. Fertility rates are falling everywhere. The median age in many countries is already over 40, well above the prime childbearing years. In some places, plummeting fertility can be attributed to dictatorial coercion: To enforce its “One-Child” policy, China has employed methods ranging from steep fines and loss of employment to compulsory sterilization and abortions. The results have been brutal: Hundreds of millions of births have been prevented, China’s median age is at 36 and rising, and the Chinese fertility rate is now 1.54 – well below the rate of 2.1 needed to maintain a steady population.

But as Last points out, the fertility rate for white, college-educated American women – a proxy for the US middle class – is 1.6. “In other words, America has created its very own ‘One-Child’ policy. It’s soft and unintentional, the result of accidents of history and thousands of little choices. But it has been just as effective.”

It is hard to overstate the demographic and social transformation this represents. It wasn’t that long ago that getting married and having children were life goals shared by nearly every American. For most of the 20th century, well over 90 percent of US adults married at some point in their lives – at one point the percentage went as high as 98.3 percent. Now, according to Pew, barely half of all adults in the United States – a record low – are married. And nearly 4 in 10 Americans say marriage is becoming obsolete.

And as more people choose not to marry, more of them retreat from childrearing. For decades Gallup has asked Americans what they consider the “ideal family size.” From the 1940s to the 1960s, roughly 70 percent said that three or more children would be best. But beginning in the late 1960s, the American “ideal” fell sharply. Today only 33 percent of Americans regard three or more kids as desirable. And in practice, one in five American women now have no children at all.

What happens to a society that increasingly turns its back on marriage and babies? In which singlehood becomes standard, and pets outnumber kids by four to one? Ready or not, America is going to find out.

(Jeff Jacoby is a columnist for The Boston Globe. His website is www.JeffJacoby.com).