It's a lovely cage. Pity about the number of rats.

How Many People Is Too Many?

By mid-October of this year, the world's third most populous nation will hit 300 million inhabitants. And thanks to America's burgeoning fertility rate, we will keep moving briskly onward, hitting 400 million in less than 40 years, by Census Bureau projections.

Is 300 million people too many -- or not enough? Wade into a discussion of population size, and you're soon up to your neck in a host of knotty issues: sex, contraception, immigration, economic justice and ecological crises....

The Coming Crunch (Needs a WSJ ticket to read)

With about 86 people per square mile nationwide now, the U.S. would seem to have plenty of room for more. Even after the next 100 million people are added, the U.S. still will have one-sixth the density of Germany, whose population is expected to stop growing within a few years.

But those averages hide disparities that could prove worrying. Even as it grows, the population is increasingly concentrating in just a dozen or so states. North Dakota is losing population, Ohio is adding a mere 20,000 people a year and heartland states like Kansas and Nebraska average fewer than 14 households per square mile.

The Center for Environment and Population, a nonpartisan research group in New Canaan, Conn., calculates that more than half the population lives within 50 miles of the coasts. In the next decade, an additional 25 million people -- half the total population increase -- will join them there.

That concentration of population is likely to result in megacities of 25 million or more as people head to them for jobs, demographers predict, raising new worries about the spread of infectious diseases and of terrorism in such dense areas. At the same time, population growth is accelerating sprawl and consumption, the byproducts of an increasingly affluent and older middle class.

The U.S. Census Bureau has been recording the steady march to the 300 million mark with an online population clock that adds one person every 11 seconds. The bureau calculates that one person is born every seven seconds, one dies every 13 seconds and one immigrates every 31 seconds....

Too many rats in a cage.