From flocks of birds to schools of fish and locust swarms, Iain Couzin (assistant professor in the Department of Ecology and Evolutionary Biology at Princeton University) outlines the mathematical models his group used to identify the behaviour of large groups of individuals when acting as part of a greater collective. There is an interesting sweet spot between attraction and repulsion. While we don't want to be stepping on each other, we can't stray too far or else we may be left behind. He also illustrates why larger groups are less vulnerable to attacks than individuals.
Taken from the Radiolab Blog.