Mobile Phone Records Reveal Largest Gathering in the History of Humanity If everyone from Tokyo and New York

Mobile phones have revolutionized the way scientists study human behavior, allowing them to watch people on a scale that has never been previously imagined.

This work has revealed patterns of sleep, mating strategies, and commuting behaviors on huge scales. It has even allowed demographers to estimate the wealth of entire countries.
One thing that this work has revealed is that collective decision-making can be very different from the decisions that humans make individually. In particular, humans in groups have a strong affinity to pick out others who are like them. So minority groups tend to stick together.