  Macroscopic behavior of scientific and societal systems results from the
aggregation of microscopic behaviors of their constituent elements, but
connecting the macroscopic with the microscopic in human behavior has
traditionally been difficult. Manifestations of homophily, the notion that
individuals tend to interact with others who resemble them, have been observed
in many small and intermediate size settings. However, whether this behavior
translates to truly macroscopic levels, and what its consequences may be,
remains unknown. Here, we use call detail records (CDRs) to examine the
population dynamics and manifestations of social and spatial homophily at a
macroscopic level among the residents of 23 states of India at the Kumbh Mela,
a 3-month-long Hindu festival. We estimate that the festival was attended by 61
million people, making it the largest gathering in the history of humanity.
While we find strong overall evidence for both types of homophily for residents
of different states, participants from low-representation states show
considerably stronger propensity for both social and spatial homophily than
those from high-representation states. These manifestations of homophily are
amplified on crowded days, such as the peak day of the festival, which we
estimate was attended by 25 million people. Our findings confirm that
homophily, which here likely arises from social influence, permeates all scales
of human behavior.
