What’s a Manhattanite to do if he or she can no longer afford Manhattan but doesn’t want to leave New York City?

Move to the Bronx, probably.

From 2001 through 2006, over 23,380 Manhattanites relocated to the Bronx, according to an analysis by The Observer of I.R.S. data. Every year, the Bronx led the three other outer boroughs in net gains of Manhattanites. That includes Brooklyn, traditionally perceived as the natural next stop in a priced-out Manhattanite’s real estate evolution.

But Brooklyn has consistently run second to the Bronx this decade as an in-city relocation destination; and Queens and Staten Island have run a distant third or fourth.

From 2002 to 2003, for instance, Brooklyn drew a net gain of 1,627 Manhattanites while the Bronx drew nearly three times as many, 4,417. Between 2005 and 2006, the last year I.R.S. data was available, 4,680 Manhattanites relocated to the Bronx and 3,731 to Brooklyn. That represents a decade-long annual peak so far for Manhattanite migration to Brooklyn; the only other time Brooklyn experienced a net annual gain of at least 3,000 Manhattanites was from 2001 to 2002 (an effect of Sept. 11?).

Speaking of peaks! From 2005 to 2006, just over 10,000 Manhattanites moved to the outer boroughs. This happened during the two years when Manhattan apartment rents and sales prices ascended to historic records—and kept ascending.

The average Manhattan apartment price was just shy of $1 million by the start of 2005, according to appraisal firm Miller Samuel; by the end of 2006, it was over a quarter of a million dollars higher.

Also, it should be noted, Manhattan’s population of children under 5 jumped 26 percent from 2000 through 2004, according to analysis of census data by The New York Times.

Perhaps the crucible of ever-rising real estate costs joined with living costs generally (education, parking, recreation, ad infinitum); and the Bronx never looked so good to so many unwilling to leave New York City but forced to leave New York County.

A few quick caveats on the data: They do not include those New Yorkers who didn’t earn enough to pay income taxes. They’re based on the addresses from which taxpayers claimed individual exemptions. Finally, remember that it’s net migration: the number of people who left Manhattan minus the number who moved in from a borough.