Finding their way to San Jose: Detroit is down, San Jose is up: what their fortunes tell us about life in America in the 21st century.

Maps and population statistics can tell us a lot about where a country is headed, both literally and figuratively. Take the U.S. Census Bureau's latest findings on American cities: They show that for the first time since 1900, Detroit is not on the list of the Top 10 most populous U.S. cities. In its place is San Jose, Calif.

Although those two cities are separated, as of last summer, by a mere 4,324 people, their new places in the pecking order speak volumes about three broad trends that have been reshaping America in the last 50 years: the decline of the manufacturing economy; the migration of Americans to the South and West; and the growth of suburbs at the expense of cities.

Detroit is in many ways the poster child for the decline of America's manufacturing centers. As the longtime hub of the U.S. auto business, the Motor City's fortunes have fallen along with those of its namesake industry.

The "Big Three" U.S. automakers--General Motors, Ford, and Chrysler (which is now owned by a German conglomerate)--have closed plants and laid off tens of thousands of workers in the past few years.

The factory closings and huge job losses have been devastating for Detroit, which was the nation's fourth-largest city 50 years ago. The city cannot collect enough tax revenue to pay its bills, forcing it to close schools and lay off city workers. Those cutbacks in turn cause others to flee the city.

"It's part of a pattern for the heavily industrialized cities," says Dana Johnson, an economist in Detroit. "There's been an ongoing dynamic here of people, middle-class people in Detroit, fleeing the city looking for better schools, better lifestyles, better services."

MOVING OVERSEAS

Detroit is hardly alone. After World War II, the U.S. was the world's leading manufacturer. Cars, televisions, toys, and clothing made in America were exported around the world, and the well-paid, unionized factory workers who made them formed the backbone of a rising middle class.

In the last three decades, the U.S. economy has shifted. Companies have moved factories--especially those in the Northeast and Midwest--overseas to take advantage of lower wages in Asia, Latin America, and Eastern Europe.

With plants closing down, it's not surprising that of the 10 cities with the largest population declines between 2000 and 2004, seven were in the Midwest. (New York remained the nation's biggest city, with 8.1 million people, followed by Los Angeles, at 3.8 million.)

Where are people who leave the Northeast and Midwest going? They are moving South and West, the second broad trend. In the 1990s, according to the census, the populations of California, Texas, and Florida swelled, along with those of Nevada, Arizona, Colorado, and Georgia. In general, people are moving to where it's warmer, less expensive, and less crowded, or where jobs are more plentiful--in growing service industries like hospitality and health care, in the technology sector, or in factories that moved from the Northeast or Midwest. (Foreign carmakers opening factories in the U.S. have tended to locate them in the South.)

In a sense, the computer chips of San Jose, the new 10th--largest city, in the heart of what is known as Silicon Valley, have replaced the sheet metal and molten steel of Detroit as the symbol of American economic power. EBay makes its home in San Jose, and Google and a host of other big technology companies are just down the road.

Many of the people leaving Detroit, however, are not going far. In an example of the third trend, the census shows that most people fleeing the city are not leaving Michigan, but are heading for the suburbs north and west of the city.

The seeds of this shift were planted in the decades after World War II, as the economy boomed and the nation's vast interstate highway system was constructed. America's growing middle class became more mobile, and living in the suburbs was that much easier.

FUTURE REVIVAL?

So although the city itself is losing population, the Detroit metropolitan area is still the nation's eighth largest, with 5.5 million people. That gives the city, which played host to the Super Bowl in January, a measure of hope that it can attract people by creating new jobs in place of the auto-factory work that has vanished.

"A lot of cities have their ups and downs," says the city's former mayor, Dennis Archer. "Now it's time for us to go back up."

With reporting by Jeremy Peters.