Thomas Malthus has been dead for 170 years, but the Malthusian fallacy – the dread conviction that the growth of human population leads to hunger, shortages, and a ravaged environment – is unfortunately alive and well:

America’s congested highways are caused by “population growth wildly out of control,” the group Californians for Population Stabilization laments in an ad.

In a new documentary, Britain’s Prince Philip blames the rising price of food on overpopulation. “Everyone thinks it’s to do with not enough food,” Queen Elizabeth II’s husband declares, “but it’s really that demand is too great – too many people.”

Overpopulation is “very serious – very, very serious,” the Dalai Lama tells a crowd of 50,000 in Seattle. Somewhat inconsistently, he also proclaims that “children are the basis of our hope,” and that “our future depends on them.”

Like other prejudices, the belief that more humanity means more misery resists compelling evidence to the contrary. In the past two centuries, the number of people living on earth has nearly septupled, climbing from 980 million to 6.5 billion. Yet human beings today are on the whole healthier, wealthier, longer-lived, better-fed, and better-educated than ever before.

The catastrophes foretold by Malthus and his epigones – some of them in bestsellers like “The Population Bomb,” which predicted that “hundreds of millions of people are going to starve to death in spite of any crash programs embarked upon now” – have never come to pass.

That is because people are not our greatest liability. They are our greatest asset – the wellspring of every quality on which human advancement depends: ambition, intuition, perseverance, ingenuity, imagination, leadership, love.

True, fewer human beings would mean fewer mouths to feed. It would also mean fewer entrepreneurs, fewer pioneers, fewer problem-solvers. Which is why it is not an increase but the coming decrease in human population that should engender foreboding.

For as Phillip Longman, a scholar of demographics and economics at the New America Foundation, observes: “Never in history have we had economic prosperity accompanied by depopulation.”

And depopulation, like it or not, is just around the corner. That is the central message of a compelling new documentary, “Demographic Winter: The Decline of the Human Family.” Longman is one of numerous experts interviewed in the film, which explores the causes and effects of one of what may be the most ominous reality of 21st-century life: the fall in human birth rates almost everywhere in the world.

Human fertility has been dropping for years and is now below replacement levels – the minimum required to prevent depopulation – in scores of countries, including China, Japan, Canada, Brazil, Turkey and all of Europe.

The world’s population is still rising, largely because of longer life spans – more people live to old age than in the past. But with far fewer children being born today, there will be far fewer adults bearing children tomorrow. In some countries, the collapse has already begun. Russia, for example, is now losing 700,000 people a year.

By mid-century, the United Nations estimates, there will be 248 million fewer children than there are now. To a culture that has been endlessly hectored about the dangers of overpopulation, that might sound like welcome news. It isn’t. No society gains when it loses its most precious resource, and no resource is more valuable than the human mind.

In 1965, the population of Italy was 52 million, of which 4.6 million, or just under 9 percent, were children younger than 5. A decade later, that age group had shrunk to 4.3 million – about 7.8 percent of Italians. By 1985, it was down to 3 million and 5.3 percent. Today, the figures are 2.5 million and 4.2 percent.

Young children are disappearing from Italian society, and the end isn’t in sight. According to one estimate by the UN’s Population Division, their numbers will drop to fewer than 1.6 million in 2020, and to 1.3 million by 2050. At that point, they will account for a mere 2.8 percent of the Italian nation.

Italy isn’t alone. There are 1.7 million fewer young children in Poland today than there were in 1960, a 50 percent drop. In Spain 30 years ago, there were nearly 3.3 million young children; there are just 2.2 million today. Across Europe, there were more than 57 million children under 5 in 1960; today, that age group has plummeted to 35 million, a decline of 38 percent.

Fertility rates – the average number of children born per woman – are falling nearly everywhere. Worldwide, reports the UN, there are 6 million fewer babies and young children today than there were in 1990. By 2015, according to one calculation, there will be 83 million fewer. By 2025, 127 million fewer. By 2050, the world’s supply of the youngest children may have plunged by a quarter of a billion, and will amount to less than 5 percent of the human family.

The reasons for this birth dearth are many.

As the number of women in the workforce has soared, many have delayed marriage and childbearing, or decided against them altogether. The sexual revolution, by making sex readily available without marriage, removed what for many men had been a powerful motive to marry. Skyrocketing rates of divorce have made women less likely to have as many children as in generations past. Years of indoctrination about the perils of “overpopulation” have led many couples to embrace childlessness as a virtue.

Result: a dramatic and inexorable aging of society. In the years ahead, the ranks of the elderly are going to swell to unprecedented levels, while the number of young people continues to dwindle. The working-age population will shrink, first in relation to the population of retirees, then in absolute terms.

A determined optimist might take this as good news. In theory, fewer people in the workforce should increase the demand for employees and thus keep unemployment low and the economy humming.

But the record tells a different story. In Japan, where the fall in fertility rates began early, the working-age population has been a diminishing share of the nation for 20 years. Yet for much of that period, unemployment has been up, not down

Far from boosting the economy, an aging population depresses it. As workers are taxed more heavily to support surging numbers of elders, they respond by working less, which leads to stagnation, which reduces economic opportunity still further.

“Imagine that all your taxes went for nothing but Social Security and Medicare,” says Longman in “Demographic Winter,” “and you still didn’t have health care as a young person.”

Gary Becker, a Nobel laureate in economics, emphasizes that nothing is more indispensable to growth than “human capital” – the knowledge, skills, and experience of men and women. That is why baby booms are so often harbingers of economic expansion and vigor. And why businesses and young people drain away from regions where population is waning.

A world without children will be a poorer world – grayer, lonelier, less creative, less confident. Children are a great blessing, but it may take their disappearance for the world to remember why.