It's worth reading the link here: the point is taht people have a tendency to segregate even for entirely arbitrary reasons, eg when they are playing members of different groups in a make-believe story for film. I've seen similar behavior on lots of film sets; if there are 'good guys' and 'bad guys' in the cast, they'll gravitate to each other at mealtimes, and likewise cast and crew tend to self-segregate as numbers increase.