The hierarchical organization of urban social overlap: How shared social spaces diverge from physical mobility
Cities are commonly characterized through population density, infrastructure, and mobility flows. These quantities describe where people are and how they move, but movement does not uniquely determine where populations repeatedly encounter one another or whether different locations participate in the same social environments. Here, we construct co-presence overlap networks across 13 Brazilian cities to distinguish these dimensions of urban organization. We introduce two complementary measures: colocation, which quantifies recurrent co-presence activity within individual locations, and co-connectedness, which measures the extent to which locations share the same co-present pairs. We find that the spatial concentration of colocation differs substantially from that of mobility, producing social centers that can differ in both number and location from mobility centers. At the network level, co-connectedness exhibits a non-random hierarchical organization in which locations preferentially share recurrent pairs with locations of similar activity. This hierarchy is systematically weaker and substantially more variable across cities than the corresponding mobility hierarchy, demonstrating that cities with similar mobility organization can differ in how recurrent social overlap is organized across locations. Finally, we show that the spatial proximity of colocation activity to social centers relative to residential centers provides a complementary spatial dimension of urban organization. Together, these results separate three features that are conflated when cities are characterized through mobility alone: movement between locations, local opportunities for social interaction, and the organization of recurrent social overlap across locations.