Whet Moser explains the GIF of Chicago’s disappearing middle-class through the work of sociologist Lincoln Quillian:

What’s most striking about Hertz’s map is the transition from 1970 onwards; when the map begins, the lowest-income census tracts are extremely concentrated. Then, as if a switch was flipped, they radiate out from the city center by 1980. (It almost looks like watching Conway’s Game of Life.) The change in those 20 years is immense. And Quillian gives a clue as to why, laying the groundwork for what was happening before Hertz’s analysis begins (emphasis mine):

Modern poor urban neighborhoods, formed after 1970 or so, thus stand in sharp demographic
contrast to poor and minority neighborhoods earlier in the century. Accounts of racial succession of neighborhoods in the 1950s indicate that neighborhoods undergoing racial transition tended to increase in population density, especially in passing through a late phase in racial succession referred to as “piling up,” in which previously white-owned homes and apartments were subdivided into smaller dwellings to accommodate the housing demands of black immigrants (Duncan and Duncan 1957). Although the affluent have always made efforts to segregate themselves from the poor, immigration into cities before about 1970 was proceeding at too rapid a pace to allow inner city neighborhoods to drop substantially in population as part of this process. Indeed, a chief reason blacks desired to exit predominantly black areas of the city before 1970 was because the housing supply in black neighborhoods was insufficient to keep up with demand (Aldrich 1975). With the end of black immigration to urban areas, poor African-American neighborhoods have changed from densely packed communities of recently arrived immigrants to areas gradually abandoned by the nonpoor. The cessation of the flow of black immigrants to the nation’s cities, and the corresponding decline in the population density of poor neighborhoods, may be one unexplored factor responsible for the change in the nature of poor African-American neighborhoods in the early 1970s that Wilson (1987) describes.

The Second Great Migration ends in 1970. To paraphrase Hunter S. Thomson, Hertz’s 1970 map appears to be the point where you can see the wave break and roll back.

Quillian’s data then picks up the narrative, which adds texture to Hertz’s map. Between 1980 and 1990, there’s a substantial leap in the lowest-income-level census tracts, then things plateau from 1990-2000. Here’s Quillian again:

There is no indication in the PSID data that stayers in black and/or poor neighborhoods experienced increases in their poverty rates in the 1970s and 1980s, except during the recession of the early 1980s. During this recession, increases in the poverty rate among the nonpoor were spatially concentrated in black moderately poor neighborhoods. Since these neighborhoods were already moderately poor to begin with, this suggests that increasing poverty rates in the early 1980s had a strong effect in increasing the number of extremely poor neighborhoods.

Quillian was writing in 1998 (here’s another paper from him in 2012, addressing similar issues), but his conclusions accurately foretell the changes you can see from 2000-2012: “Neighborhoods in transition to high-poverty status empty first of whites, then of many middle-class blacks, leaving more-disadvantaged and less-populous areas. The overall result is that high-poverty neighborhoods have been becoming geographically larger and less densely settled.”

So some of these neighborhoods that changed over to high levels of poverty aren’t necessarily the result of increasing number of poor people but rather the departure of higher-income and white residents. They may be poor neighborhoods but they are not necessarily dense because few people of any background (regardless of class and race) are moving in.

Another thought: some conversation about white flight focuses on the 1950s and 1960s when whites moved to the suburbs due to (1) policies that helped make the suburbs more attractive (interstate construction, new rules about mortgages that made home purchases available to more Americans plus (2) continued waves of the Great Migration of blacks to Northern cities. All this is true but this map is a reminder that the processes affecting poor neighborhoods continued from the 1970s to 1990s. It wasn’t until the 1980s that academics started writing important books like this, whether from William Julius Wilson or Paul Jargowsky.

Of course, a key question is how much this is still happening today. Can poor neighborhoods spread even further as better-off urban residents and suburban residents move to wealthier pockets while lower-class and poorer residents are left in emptying out locales? The process may not be over yet and it is hard to find cases where truly poor neighborhoods from recent decades made substantial turnarounds.