PHILLIP LONGMAN is the most important man you've never heard of in Washington.

A senior fellow at the liberal New America Foundation, Longman specializes in demography. If you're a romantic, demography is the science of love, writ large. If you're a cynic, it is the sausage factory of civilization. Whatever your disposition, demography is, if not destiny, then a subject of paramount importance. In the long run, no weapon, no technology, no economic system is more powerful.

Longman has spent many years studying demographic trends, and the conclusions are unsettling. As he writes in his 2004 book, The Empty Cradle, birthrates in America and around the world are declining beneath sustainability; population growth is slowing and, unless the trends of the last 200 years change, will soon bring about population decline--and with it, potential shifts in global prosperity and power.

Forget domestic politics and international relations: Fertility is the thing. As Longman explains, it's the grand unified theory of everything. As fertility rates decline, populations, then economies, then military power, then world influence, diminish.

This is a bit counterintuitive. As Longman notes, everywhere you look there are signs of overcrowding. More traffic, more housing sprawl, more strip centers, more kids applying to college. It looks as if the world is bursting at the seams.

There's some truth to that. There are 6.5 billion people today, and that number is increasing every year. But according to demographic estimates, the world's population will peak somewhere between 9 billion and 12 billion in the next 75 years--give or take--and after that will precipitously decline, while the average age of the population gets more and more advanced.

The key concept is that of fertility rates. The "replacement fertility rate," which is to say the number of children the average woman needs to bear for a population to sustain itself, is 2.1.

Global fertility rates have been declining for a long time. Today, they're half of what they were in 1972. Fifty-nine countries (accounting for 44 percent of world population) have fertility rates below replacement levels. The United Nations projects that by 2050, 75 percent of all countries will fall below replacement levels.

Let's be clear: Even as fertility rates decline, absolute population size continues to increase because of demographic momentum. But as the fertility rate decreases, the rate of population increase slows--until you dip beneath the 2.1 line. Suddenly, the pool of potential new parents becomes too small to sustain the population. After about 30 years, the grandparents begin dying off.

"By then, the momentum of population growth is lost," writes Longman, "or, more precisely, is working in the opposite direction with compounding force."

You can already see this trend in the United States. We have the highest fertility rate in the industrialized world (2.09), but this mainly reflects the contribution of our immigrants, who reproduce at a higher rate than natives.

As Colgate economist Michael Haines has shown, American fertility rates have been falling steadily for 200 years. In 1800, the fertility rate among white Americans was 7.04; by 1998, it was 2.07. This decline was interrupted by only a single period of increase: the Baby Boom. In 1940, the fertility rate was 2.22; in 1950, it rose to 2.98; in 1960, it rose further still to 3.53. But by 1970, it fell back to 2.39 and has been headed south ever since.

The fertility rate for black Americans is in steeper decline. In 1850, it was 7.90. Blacks, too, experienced a Baby Boom between 1940 and 1960, but by 1998, their fertility rate was 2.17 and falling fast. Hispanics are the only American ethnic group significantly above the replacement level, because Latin American immigrants bring with them higher fertility rates. After a few years in the States, they begin regressing to the mean: Between 1990 and 2001, America's Hispanic birthrate fell 10 percent.

Immigration might seem like a solution to our demographic woes, but that's a mirage. As the U.N. report "Replacement Migration" explains, to keep the current ratio of workers to retirees in America, we'd need 10.8 million new immigrants every year until 2050, at which point the U.S. population would be 1.1 billion, 73 percent of whom would be immigrants arrived since 1995 and their descendants. As a sociological matter, that's an untenable situation.

(It's also unlikely. Manhattan Institute scholar Tamar Jacoby persuasively argues that falling fertility rates and rising median ages in Latin America will probably cause the source of immigration to dry up long before our need for bodies is satisfied.)

So what happens next? According to the U.S. Census, between 2005 and 2025, America's over-65 population will increase by 72 percent, because as fertility decreases, the existing population gets older. By 2050, about 20 percent of Americans will be over 65; there will be 13 million more senior citizens than there will be children under 14.

An older, contracting population is a harbinger of dark times. In the modern welfare state, the cost of caring for the elderly is largely shifted to the government. Combine an increasing population of seniors with an increasingly expensive state pension and health-care system, and you have a portion of the budget that must grow ever larger. The options: slash benefits, overhaul the system, or raise taxes. As Longman explains: "Younger workers, finding that not only does the economy require them to have far higher levels of education than did their parents, but that they must also pay far higher payroll taxes, are less able to afford children, and so have fewer of them, causing a new cycle of population aging."

In other words, the further the fertility rate falls, the greater the incentive for people to have fewer children.

Capitalism is, historically speaking, a relatively new contraption, but recent experience suggests that capitalism and falling populations don't mix particularly well. Consider Japan and Europe. Japan's fertility rate is 1.34, 17 percent of its population is over 65, and its economy is a shambles. By 2050, Japan will lose a seventh of its population, and the percentage of citizens over 65 will increase from 17 percent to 32 percent. Italy - never, and certainly not now, a model of smoothly running capitalism--will lose 13 percent of its population, while the proportion of those over 65 will double to 35 percent. In Russia, which is already losing 750,000 people a year, that future is now.

In the coming years, the United States will struggle to avoid this fate. Our declining fertility is, literally, a matter of life and death.

Jonathan V. Last is online editor of The Weekly Standard and a weekly op-ed contributor to the Philadelphia Inquirer. This essay originally appeared in the May 21, 2006 edition of the Philadelphia Inquirer.

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