The simple metaphor of a bathtub is widely used to capture these dynamic relations within human migration. The level of population in a city is like the level of water in a bathtub, held steady by an inflow from the faucet and an equal outflow from the drain. A rising water level could be evidence of faster inflow, or it might be that the drain has been clogged. Similarly, a falling water level might be attributed to an inflow made slower, signifying a weakened preference by human movers to arrive, but it just as well could be due to a more open drain.
Relatively normal outflow combined with weak inflow into San Francisco would also result in a demographic decline of African Americans. Residents living in these neighborhoods wouldn't notice the lack of blacks moving into the area. They would notice those who left.
Blaming out-migration for population loss is rational. Expecting zero out-migration is irrational. People leave every city, no matter how cool and economically vibrant. Save some sort of acute crisis, lack of in-migration is the typical reason for dwindling numbers (leaving out deaths and births). Turns out, San Francisco is no exception:
The story of San Francisco’s declining black population is characterized more by a lack of in migration than an unusual amount of out migration. Just about 1 in 10 African Americans who live in San Francisco leave the city every year. This is not much greater than for Whites or Hispanics. This out migration is in some ways positive, in part representing an ability to leave the city that was not possible in the days of stronger housing discrimination.
If San Francisco is a bathtub, then the flow out the drain for African Americans is similar to other groups. The flow into the basin is unusually low. Mystery, or what should have been a mystery, solved.
"The flip side of this out-migration issue is: Why is there no in-migration?" [James McCray Jr., director of the Tabernacle Community Development Corp.] continued. "Why are there no African Americans or Latinos coming here to contribute to this great city?"
I'm surprised that someone would ask such a question. I wish more people would engage in this kind of inquiry instead of fixing up a place in order to keep residents from leaving. Out-migration happens. Nicer cities wouldn't have put the kibosh on sprawl. But they could have encouraged more newcomers. The concern about out-migration is misplaced, a misreading of the facts. The tub tells all.