The researchers then used demographic information to map the BMIs. The map below is based on the study’s follow-up observations from 2009-2012. The red dots indicate areas with “unfavorable” BMIs that were proportionally higher

than in the rest of the city. In contrast, the blue dots show areas with “favorable,” or proportionally lower, BMIs.

Ecole Polytechnique Fédérale de Lausanne

In both the initial study and the follow-up, the researchers noticed a stark divide between working-class, less-educated residents clustered in the city’s western edge—who had higher BMIs—and wealthier, well-educated residents farther east, whose BMIs were lower.

These findings appear consistent with research that links obesity to poverty and low-income neighborhoods. But, surprisingly, when the authors adjusted the data for neighborhood-level median income, the results did not change significantly. Even after removing income from the equation, Lausanne’s western population was still bathed in red (as shown on the map below). This suggests that a higher BMI was not the result of neighborhood income alone. Nor was it the direct result of other factors, such as age, race, education, smoking, or alcohol consumption—all of which were adjusted for in the study.

Ecole Polytechnique Fédérale de Lausanne

In the end, the study posits that urbanism could be one of the missing links between obesity and this location-based divide. According to the study’s lead author, Stéphane Joost, large roads, crowded highways, and metro lines have isolated working-class communities from “places that could be very healthy for them”—namely, green spaces. Joost also notes that this kind of isolation tends to limit residents’ mobility and hinder their access to healthy food or supermarkets. (It is worth noting, however, that making healthy food accessible does not always lead to a change in consumer behavior.)

But access isn’t the only impediment to health. Joost also believes that something called “spatial dependence”—or the idea that your behavior is influenced by your neighbors—could be partly responsible for the high BMIs in Lausanne’s western region.

Back in 2007, an analysis of data from the Framingham Heart Study found that obesity occurs in clusters, based largely on social ties. Most notably, the analysis discovered that a person’s chance of becoming obese increased by 57 percent if his or her friend was obese. This pattern continued for up to three degrees of separation. Joost speculates that a similar phenomenon is at work in Lausanne. Since many working-class residents live in subsidized housing, it is quite possible that these residents are promoting a less healthy lifestyle within their neighborhoods.

Of course, residents also cluster based on where they can afford to live, and the fact still remains that the built environment of Lausanne has forced working-class communities into areas that lack the sort of green space and amenities that foster public health. “It’s not by chance that people with modest incomes live where they live,” says Joost.