Most of us have grown sufficiently used to the idea of gang colors that we don't bother to ask why they exist. But, if you think about it, they're paradoxical: Why would people who belong to groups that sell drugs or commit crimes go out of their way to advertise their membership in those groups? A new paper, by Andrew Mell, an economist at Nuffield College, Oxford, attacks that conceptual problem.

He does so by drawing on evolutionary theory and the related issue of coordination between agents. If you're a criminal, one of...