City voters were more racially diverse and poorer than the suburban electorate, and thus less able to offer low property taxes or high-quality public services. If border residents also fled the city as black migrants arrived, even though black enclaves were miles away, these departures signaled a concern about broader city finances rather than a dislike of immediate black neighbors.

Not surprisingly, houses on the suburban side of the border have always been a little more expensive than their city counterparts. Using data on 100 border neighborhoods in the 1960s and 1970s, I find that this cross-border housing price gap grew by a few percentage points — to a 7 percent suburban housing price premium from a 5 percent premium — as black migrants flowed into the city, even though new black arrivals lived miles away. Households in these areas were motivated by concerns about how a changing local electorate would affect property taxes and service levels. In fact, for this set of households, what mattered most about the new Southern arrivals crowding into neighborhoods across town was not their race but their lower levels of income.

That doesn’t mean racism wasn’t a motivating factor. For the third of white households near a black enclave in 1940, concerns about new black neighbors was indeed a primary motivation. And those households moved out of the city at a higher rate than others, contributing more than a third to the white exodus. But for the remainder of urban whites, most of whom never interacted with a black family, leaving for the resource-rich suburbs was an economic calculus, one that was accelerated by the steady stream of poor migrants, both white and black, into central cities.

Just like Trump voters in 2016, different people in the same group — white urban households — took the same action, around the same time, but for different reasons. To complicate the picture, few of them left personal accounts, and they may not have been able to articulate exactly why they moved. We are left reconstructing the pieces through careful detective work. In my own work, I have found that Poirot is often right: Each suspect wielded his own knife.