People in the demographics business like to think of themselves as the only futurists you can trust. They've got a point: if you want to know how many 21-year-olds there will be in 2027, just count the number of infants living today. Absent a catastrophe of biblical proportions, you'll come up with a pretty good prediction.

What demographers admit they're not so good at is anticipating change. (For example, they were terrible at projecting the impact of birth control.) At the height of the "population explosion" hysteria four decades ago, few believed that birthrates could fall so far and so fast that the population of a major country like Russia would actually start shrinking (as it did about 14 years ago). Germany's tipping point seems to have arrived in 2002, and Japan's in 2005.

So what are we to make of the moment, projected by the U.S. Census Bureau to arrive this month, when the population of the United States reaches 300 million, behind only that of China and India? Demographics is simply the arithmetic of culture and values—it only quantifies, it doesn't explain. Is 300 million a good thing? A bad thing? Thinking about that number provides an opportunity to talk about where we're headed and what makes us tick.

Readers who remember November 20, 1967, when the population of the United States passed 200 million, may recall the predictions of Paul R. Ehrlich. In The Population Bomb, in 1968, he foretold "certain" mass starvation by 1975 because of population growth. "The battle to feed all of humanity is over," Ehrlich's first sentence read. "In the 1970s and 1980s hundreds of millions of people will starve to death in spite of any crash programs," he declared. At best, North America and Europe would have to undergo "mild" food rationing within the decade as starvation and riots swept across Asia, Latin America, Africa and the Arab countries; at worst, the turmoil in a foodless Third World would set off a series of international crises leading to thermonuclear war.

Of course, things didn't quite work out that way. The problem in the United States is obesity. Even in places like Somalia and Sudan, famines have been intractable not because of any global lack of food, but because the food has not gotten to the people who need it—too often because corrupt regimes have withheld it as a means of political control. Nonetheless, Ehrlich's misjudgment sold more than three million copies, and the phrase "population bomb" entered the vocabulary.

That's why some people find it hard to wrap their minds around the big news in demographic circles today. It's not catastrophic population growth. It's catastrophic population shrinkage.

Yes, shrinkage. True, the total global population has not yet finished increasing. But nearly half the world's population lives in countries where the native-born are not reproducing fast enough to replace themselves. This is true in Western Europe, Eastern Europe, Russia, Japan, Canada and the United States. It's also true in much of East Asia, pockets of Latin America and such Indian megacities as New Delhi, Mumbai (Bombay), Kolkata (Calcutta) and Chennai (Madras). Even China is reproducing at levels that fall short of replacement.

Typically, a couple has to produce about 2.1 children to replace themselves, allowing for death among the young. Even in traditionally Catholic countries in Europe, the birthrate has dropped to shockingly low levels in the last two generations: 1.3 in both Italy and Spain in 2005. In metropolitan Tokyo, the rate dropped to 0.98. In Hong Kong and Macau, it hit 0.96 and a hitherto unthinkable 0.84, respectively, the latter the lowest on record. Few demographers ever dreamed that in the absence of war, famine and pestilence—in fact, as a result of urbanization, development and education—birthrates would drop so dramatically. No one knows where the bottom is. Keep this up, and eventually your civilization will disappear.

The United States' population is growing at the rate of almost 1 percent per year, thanks in part to immigration and its secondary effects. Not only does the United States accept more legal immigrants as permanent residents than the rest of the world combined, but these recent arrivals tend to have more children than established residents—until, as their descendants attain affluence and education, the birthrates of these Americans also drop below replacement levels. Overall—that is, counting both immigrants and the native-born—the United States has a replacement rate of 2.03.

Nearly half of the nation's children under 5 belong to a racial or ethnic minority. The face of the future is already in our schools: our kindergartens now prefigure the country as a whole, circa 2050—a place where non-Hispanic whites are a slight majority. High-achieving school systems are already adapting: in Fairfax County, Virginia, for example, where 93 percent of all high-school graduates go on to post-secondary education, programs that teach English as a second language accommodate more than 100 native tongues, including more than five flavors of Chinese.

Few Americans quarrel with the idea of legal immigration. Not only is it part of the national narrative, but we're especially delighted when these immigrants help create companies such as Intel, eBay and Google. Of course large numbers of people showing up without paperwork stirs passions, as attested to this year by the rise of the Minuteman Project of civilians patrolling the border with Mexico, the deployment of National Guard troops to do the same, the protracted debate over immigration bills in Congress and the stark demonstrations related to the legislation.

However that debate is resolved, it's probably worth noting a few historical assimilation practices in the United States. First, this country has a long and distinguished record of taking illiterate peasants from every desert, tundra and bog and turning them into overfed suburbanites in three generations or less. Second, new immigrants usually do not marry outside their ethnic group; their adult children do, with some controversy, and their adult grandchildren can't remember what the fuss was all about. Finally, the traditional deal America has offered immigrants is: work, pay taxes, learn English, send your kids to school and stay out of trouble with the law, and we'll pretty much leave you alone.

One fortuitous result of the enormous wave of immigrants coming to the United States is that the median age here is only a little over 35, one of the lowest among the world's more developed countries. This country also has the most productive population per person of any country on the planet—no matter how you measure it, and especially compared with Japan and the members of the European Union.

This is crucial to everyone who plans to retire, because once you do, you'll want a bunch of young, hardworking, tax-paying people supporting you, whether directly, through family contributions, or indirectly, through Social Security or pension programs. Unless you're rich enough to live off your investments, there is no alternative. As it happens, retirement is on the minds of many, and not just in the United States.

Today, virtually every developed country's population is older, typically, than that of just about every human society before 1950.

Much has been written about how hard it's going to be for European countries and Japan to support their aging populations at the generous level of social services to which previous generations have become accustomed. But global graying offers an even more formidable challenge to less wealthy countries.

By 2025, according to the United Nations and the U.S. Census Bureau, China will account for less than a fifth of the world's population, but almost a fourth of the world's people over 65, many of them in China's poorest areas. That means that in less than 20 years, large parts of China will have to support very aged populations on very low average incomes.

This is a problem Americans should be grateful they don't have, for all sorts of reasons.
First, China's version of Social Security is a colossal mess, even by the standards of the American and European systems. It covers only about a sixth of all workers. Its unfunded liabilities appear to exceed the country's total gross national product—maybe by a lot.

Second, the ages-old Chinese practice of adult children supporting their parents is coming undone. Traditionally, that obligation has passed through males; daughters are supposed to help support their husbands' parents before seeing to their own. But there's a problem here: because of Chinese population control, a woman turning 60 in 2025 will likely have had fewer than two children in her lifetime, and the odds are about one in three that she will not have borne a son.

If you're old and poor and you can't rely on either your government or your grown children for support, you have to keep working. In China, this does not mean greeting customers at Wal-Mart, much less answering the technical support line at Dell. Many of China's elderly barely have a primary school education, live in rural areas and haven't had the food and health care that would allow them to be vigorous in their old age. Nonetheless, the only work available to them is farming, which without mechanized tools is a tough row to hoe.

It's not a pretty future. Even if China's economy continues to grow by 8 percent per year, every year, for two decades—a scenario that is difficult to construct—the older generation is in big trouble. "China's outlook for population aging," political economist Nicholas Eberstadt writes, is "a slow-motion humanitarian tragedy already underway."

But not even China is as bad off as Russia. Americans talk about age 40 being the new 30 and 80 being the new 60, but in Russia, 30 is the new 40. Since the 1960s, just about each new generation of Russians has become more fragile than the one that preceded it. Every year, 700,000 more Russians die than are born.

"Pronounced long-term deterioration of public health in an industrialized society during peacetime is a highly anomalous, indeed counterintuitive proposition for the modern sensibility," Eberstadt writes. "Nevertheless, over the four decades between 1961-62 and 2003, life expectancy at birth in Russia fell by nearly five years for males." What's more, he notes, this increased mortality was concentrated among working-age men: "Between 1970-71 and 2003, for example, every female cohort between the ages of 25 and 59 suffered at least a 40 percent increase in death rates; for men between the ages of 30 and 64, the corresponding figures uniformly exceeded 50 percent, and some cases exceeded 80 percent."

Demographers and public health specialists are at a loss to explain these awful numbers, though such obvious factors as diet, smoking, drinking and sedentary lifestyles certainly enter in. One mystery in the "ongoing Russian health disaster," Eberstadt adds, "is that the problem looks to be worse than the sum of its parts: that is to say, death rates are significantly higher than one would predict on the basis of observed risk factors alone."

Whatever the answer, the future is grim: a Russian man has barely a fifty-fifty chance of making it to age 65 while, in the developed world, the over-80s make up the fastest-growing portion of the population.

Are you feeling any more comfortable with America’s healthier, younger 300 million by now? Wait, there's more.

At the rate ethnic Germans are not reproducing, they will probably lose the equivalent of the entire population of the former East Germany by mid-century. Who will fill up the rest of the country? Immigrants from Muslim countries is the odds-on bet. But as last year's riots in France and subway bombings in England demonstrate, Europe is not having a lot of luck assimilating its immigrants. In the Netherlands, for example, where nationality is based on ancient ties to family or land, concepts that seem unremarkable in North America—such as "Moroccan-Americans" or "Moroccan-Canadians"—simply have no meaning. The Dutch language offers two words: autochtonen ("us") and allochtonen ("them"); the Dutch people are still working to find ways to incorporate the latter into the former.

And yet: just about the time you start feeling comparatively good about living in a nice, young, healthy, assimilationist United States, you get smacked upside the head by the mind-boggling and peculiarly American problems this country's growth creates.

One is that to accommodate our growth of almost 1 percent a year—about 2.8 million new Americans annually—we have to build the equivalent of one Chicago per year. That's not impossible. Lord knows we have enough developers eager to do the job. What's more, if you fly across this country and look down, you will see that it includes a lot of emptiness. If you are among those people stuck in endless traffic jams from Boston to Richmond and from San Diego to Santa Barbara, you may find this hard to believe, but only 4 percent of all the land in the contiguous United States is urbanized, and only 5.5 percent is developed.

The problem is that we want to build these new Chicagos in nice places—the Mediterranean climes of California, or the deserts of Phoenix and Las Vegas, or near the oceans or the Gulf of Mexico. (More than half the American population already lives within coastal counties of the Atlantic, Pacific, Gulf of Mexico or Great Lakes.) The mountains will also do, which is why you see explosive growth near Virginia's Blue Ridge, the Gold Country of the California Sierra and even the Big Sky Country of Montana.

Unfortunately, in our search for new utopias we don't merely pave over paradise; we massively annoy the planet. Natural disasters are getting more expensive not only because the weather is getting worse but also because we keep putting our new Chicagos in harm's way.

What are the morals of these recitations?

Two leap to mind.

The first is, whenever you start thinking that this country is screwed up beyond redemption, it pays to travel beyond our borders. It's amazing how often the not-so-wonderful realities that we think of as terrible problems constitute other people's dreams.

The second is, demographics may not be destiny. But the numerical study of who we are and how we got that way does have a refreshing habit of focusing our attention on what's important, long-term, about our culture and values—where we're headed, and what makes us tick.