If you live in Manhattan, you most likely rent a three-room apartment built before World War II that has one bedroom, and that you moved into this decade. If you live in Brooklyn, you likely rent a four-room apartment with two bedrooms; the size, age and your personal history with the place isn’t much different from what it would have been in Manhattan.

And, oh yes, you more often than not share the apartment.

Census Bureau numbers out last week show a New York City that’s not surprising to anyone who’s undergone the arduous process of a New York apartment search: We live small, transiently and expensively; and it hasn’t changed much, even in this era of Wall Street bonus records and $50 million apartments at the Plaza and post-Giuliani gentrification.

Little wonder the solidly middle class are fleeing the city. Rather than sink over one-third of its monthly income into housing costs—whether through mortgages or rents—a household making between $40,000 and $60,000 a year generally exits the city, leaving the very wealthy and the working class behind to further stratify the Big Apple.

In fact, the average annual income of households that left the city in 2005 was $72,443, according to a report from City Comptroller William Thompson Jr. analyzing the Census numbers; the income of the households that stayed averaged $66,509.

Of all the apartments in Brooklyn, the 2006 Census Bureau estimates that over 26 percent have four rooms. The next most common is three rooms, the most prevalent size in Manhattan, with over a quarter-million apartments.

The next most common Manhattan size is four rooms, with 192,616. The average household size for both owner-occupied and renter-occupied apartments in Manhattan is a little over two people; in Brooklyn it’s about three. So, we’re either doubling up a lot through coupling or through extremely cramped roommate situations.

Back in 2000, the last year the Census released such estimates, the three-room dominated Manhattan as well, with the four-room a distant, distant second.

More residents moved into their Manhattan homes, small or otherwise, from 2000 through 2004—220,453—than moved into them in either the 1990’s or the 1980’s. In Brooklyn, these numbers were almost identical in proportion—most people moved from 2000 through 2004, or in the 1990’s.

This could be a function of a couple of things: the creation of new housing, particularly condos, during this decade; and also the increasingly transient nature of New York City’s population, a transience spurred in large part by housing costs.

Twice as many people move out of the city than migrate here each year, according to the report from Mr. Thompson’s office. That report concluded that about 4 percent of the five boroughs’ population turned over in 2005, not including the natural changes caused by deaths and births.

And most of the people leaving aren’t who you might think: Sure, the rich stay—the city’s a playground for many right now—but the relatively lower-income households also stick around.

The likeliest households to exit New York were those earning between $40,000 and $60,000 annually—this represents the solidly middle-class in a city where the median household income is $40,000, according to a New York University study using data from 2005. Though these made up only 17 percent of non-elderly households in 2005, they accounted for 22 percent of those households that left.

Interestingly, the next likeliest income bracket to flee the city was households making $140,000 to $249,000—suggesting that perhaps it’s not just income but what you get for that income that’s dictating the exodus.

Those households earning less than $40,000 annually, and those earning much more (over $250,000), were the likeliest to stay.

Any middle-class—or even upper-middle-class—flight is understandable, given the chunks of income that New Yorkers pay on housing.

Of the 110,663 Manhattan homes with a mortgage, nearly one-fourth spend at least 35 percent of the household’s monthly income on housing costs, according to the Census estimates. Of the 562,469 occupied rental apartments in Manhattan, over 34 percent spend at least 35 percent of the household’s monthly income on rent. Another 8.4 percent spend 30 to 34.9 percent.

Of the 182,226 Brooklyn homes with a mortgage, over 46 percent spend at least 35 percent of the household’s monthly income on housing costs. Of the estimated 590,843 rental apartments in Brooklyn, nearly 42 percent—you guessed it—pay at least 35 percent of the household’s monthly income on rent.

So maybe the middle class have it right, sad as that might be for the city’s future: Why not take your annual incomes that just about anywhere else in the United States would buy decent-size homes and move elsewhere for just that? Isn’t that why God—or the State of New York—invented the Metro North and the LIRR?

Meanwhile, the very rich and the working-class are sticking around the increasingly expensive city, economically stratifying our little bulkhead on the Hudson ever more.