America’s big cities were once engines of upward mobility, but they no longer help people climb the economic ladder as they used to. A recent study from Stanford University, using remote sensing and administrative data, finds that since the mid-20th century, urban growth has stopped boosting intergenerational mobility.

Cities have long been places of both opportunity and inequality. The researchers looked at how the size, density, and connectedness of a person’s birthplace affected their chances of moving up in life over the past century. Their findings suggest that while urban growth once increased mobility, it now holds people back.

Flipped pattern

Between 1920 and 1950, higher population density was linked to greater social mobility. But from 1950 to 1990, the pattern flipped—denser cities became less helpful for economic advancement. The same shift happened with inequality: in the early 20th century, urban expansion reduced it, but in the late 20th century, it widened the gap.

One likely reason is the decline of social capital. As cities grow, people’s ties to their communities weaken. Fewer people engage in civic life, join local groups, or maintain strong friendships. This breakdown in social bonds, the authors argue, may explain why today’s cities no longer offer the same opportunities they once did.

“Our findings confirm that contemporary cities, particularly population-dense and expansive ones, are indeed divisive forces—acting as centers for income and wealth generation but failing to deliver equal opportunities for economic mobility,” the researchers conclude. “Perhaps surprisingly, this polarizing dynamic is a recent phenomenon. In the past, the most urbanized regions performed well in terms of income creation and equality of opportunity. Our analysis supports the hypothesis that the mid-20th century marked a pivotal shift toward more unequal and less inclusive patterns of urban growth”?