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Per-capita income in the D.C. region is $61,743 as of 2012. That sounds pretty high. Between 2008 and 2012, per-capita income in the region increased by 5.4 percent. That sounds pretty good.

Except that incomes weren’t the only thing rising. According to a new report from the Center for Regional Analysis at George Mason University, the price of goods and services was rising, too. In fact, it was rising faster than incomes—-so much so that real incomes actually declined by 4.1 percent between 2008 and 2012.

Compared to the other big metropolitan areas in America, D.C. doesn’t stack up very well in this regard:

According to the report, the D.C. area experienced the third-biggest increase in prices over this time period, behind New York and San Francisco-Oakland. But incomes in those cities rose faster than in D.C., leaving New York with a 1.8 percent drop in real incomes and San Francisco-Oakland with a 0.8 percent gain.

So what accounts for the big jump in prices in the D.C. area? In a word, rents.

In 2008, according to the report, D.C.-area rental costs were 48.1 percent above the national average. In 2012, they were 69.6 percent higher. That’s a huge increase—-the biggest, in fact, of the 15 largest metro areas, and the third-biggest of all 381 metro areas studied, behind just San Jose-Sunnyvale, Calif., and State College, Pa.

This is what people are really talking about when they say that affordable housing in D.C. is dwindling. It’s not that there are fewer overall units, or fewer subsidized units. It’s that rents are shooting up, and incomes aren’t shooting up to match. A new nationwide study from the National Employment Law Project found that in the recovery from the recession, low-wage jobs are proliferating, but the middle-income jobs lost during the recession haven’t come close to being replaced. That’s why, in D.C., you find homeless residents who work jobs but still can’t afford housing.

And it’s why, ultimately, city investments in new affordable housing simply won’t be enough to make the city affordable for all of its residents. To accomplish that, the growing gap between incomes and costs will need to be bridged, with higher incomes, a slower rate of price increases (particularly rents), or ideally both.

Chart from the CRA study