A recent paper (of which one of us, Carlo Ratti, is a co-author) uses anonymized data from telecommunications networks across Europe to explore how human networks change with city size. The results are striking: in large cities, people not only walk faster (a tendency recorded since the 1960s), but they also make—and change—friends faster. This phenomenon is likely rooted in the fact that, in accordance with West’s findings, the total number of human connections increases with city size. London’s eight million inhabitants regularly connect with almost twice the number of people as Cambridge’s 100,000 residents. This increasing exposure to people—and hence to ideas, activities, and even diseases—could explain the impact of city size on socioeconomic outcomes. But another tendency is also consistent across cities of all sizes: people tend to build “villages” around themselves. This behavior is quantified as the networks’ “clustering coefficient"—that is, the probability that a person’s friends will also be friends with one another—and remains extraordinarily stable across metropolitan areas. Simply put, humans everywhere are naturally inclined to live within tight-knit communities.