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This rather alarming chart comes from the latest monthly economic report from the office of the D.C. Chief Financial Officer. No longer can city leaders boast that the District is gaining 1,000 residents a month. Instead, the city’s population growth, so important to continuing its economic revival, appears to be returning to the sluggish numbers of the pre-2009 era.

What’s causing this slowdown? According to the CFO’s office, it’s a “sharp decline in net domestic inmigration.” In other words, fewer Americans are moving to D.C. (or more residents are leaving). The report states that the U.S. Census Bureau didn’t provide a reason for the decline, but it’s not hard to speculate. Federal employment boomed during the recession years, as the stimulus and other initiatives bulked up the D.C.-area workforce; since then, it’s tanked. Federal employment growth went from positive to negative in late 2011. It’s no surprise, then, that the city’s population growth started to drop at the same time.

The more interesting question is who’s coming, and who’s leaving. We can find some of the answers in another recent report from the CFO’s office.

The conventional wisdom is that D.C. has been flooded with well-heeled 20-somethings in recent years, while low-income families have been pushed out by rising housing costs. But the numbers tell a different story.

According to an examination of tax data by the CFO’s office, there’s a sharp income divide when it comes to demographic change among D.C.’s single and married adults. And it’s almost the opposite of the commonly held belief.

As the chart above shows, the city has gained large numbers of low-income single people since 2001, and very few wealthier single people. On the flip side, the gains in married residents—both with and without children—came almost entirely from middle-class and wealthy households.

These differences appear considerably less dramatic when displayed as proportional increases, rather than absolute gains. But it’s still clear that the growth in married residents comes from the upper end of the income spectrum, much more so than among singles.

It’s worth noting, though, that these numbers extend only through 2012. Not only do they not take into account declining population growth in the past couple of years, but they also don’t reflect the ever-higher housing costs of those years, which may have shifted the balance slightly more toward wealthier residents overall.

“One should be cautious about drawing firm conclusions about DC population trends based solely on the most recent Census estimate because data is revised as more information becomes available,” the CFO’s office advises. “But it is fair to conclude at this point that population cannot be assumed to continue to increase in the future as it has in the recent past.”

For a city that’s built its expanding retail base, bigger budgets, and improving reputation on a rising population, that’s a trend that ought to raise serious concern.

Charts from OCFO