He assembled rough estimates of median income by race in (taken together) Kent, Renton, and Federal Way, the largest cities in South King County, and in Bellevue and Redmond, the largest Eastside cities. In the former cities, white and Hispanic household incomes are somewhat lower than the Seattle medians. But black median income there is nearly 30 percent higher than in Seattle, and the Asian median is nearly 20 percent higher.

The differences between Seattle and Bellevue/Redmond are even more pronounced. Everyone earns more money over there (and needs it to live there): Overall and white median household incomes are about $85k a year. But Asian and African American households earn a lot more, nearly twice as much: about $95k and $70k respectively.

Immigration doesn’t explain the differences. About a quarter of the households labeled African American in both Seattle and the county are African immigrants, versus just 3 percent nationwide. A smaller share of Asian residents in Seattle – 65 percent – are immigrants than in King County and the United States.

And foreign-born doesn’t necessarily mean low income; the whopping Asian median income in Bellevue and Redmond reflects all Microsoft’s engineers and other software workers from India, China, and neighboring counties.

So why is Asian income so low in Seattle? Perhaps a larger share of immigrants here are refugees just starting to get a stake? But resettlement agencies now tend to place new arrivals in the south county, especially Kent, Tukwila, and Sea-Tac, where rents are lower. And according to Felt’s rough numbers, median incomes are higher there.

Length of time in the United States doesn’t seem to explain it either. Similar shares of Seattle and U.S, Asians arrived pre-1990,and in the ’90s and ’00s. 

When I asked her, city staff demographer Diana Canzoneri uncovered a few other possible factors deep in the census data. Only half of Seattle’s adult Asian residents are married, versus two-thirds nationwide. A larger share in Seattle — though still just 17 percent — are divorced, separated, or widowed. More than a third again as many – 37 percent – are “linguistically isolated.” Larger shares are younger than 25 or older than 65. More of them are in college or grad school — 15 percent in Seattle versus 11 percent nationally though their effect on income indices may be minimal. (Students living in dorms aren't counted in the households survey.)

Put these all together and a composite portrait starts to emerge: More of Seattle’s Asians are students or just out of college, or elderly, or perhaps single parents. A larger share have limited English proficiency.

But most of these cohorts are fairly small, and together don’t seem to explain the gaping disparity in income. Comparable data from the rest of the county are not so readily available. And one other factor is less quantifiable: the centrifugal effect of economic mobility.

As immigrant groups gain more income and wealth, they’ve tended to move out from city to suburbs, seeking more space and safety and better schools. (Then their offspring or descendants move back seeking urban amenities.) Vietnamese, Ethiopian, and other newer groups are no different. Those unable or unwilling to move from the modest, often subsidized, housing where they first landed stay behind; they’re also more likely to be unable to work, or to make much money when they do.

There’s much yet to be explored in these demographic and economic faultlines.