Theorists like to do a lot with a little. And not just because simple theories seem more elegant: we deepen our understanding when we learn that disparate phenomena are linked together. In physics, for example, the theory of thermodynamics showed us the relationship between mechanics and heat. In economics, the theory of the firm showed us that, across industries that look nothing alike, a simple principle helps explain the organization of economic activity. Of course, there is no guarantee that the disparate phenomena we suspect are linked actually are. Particle physicists continue to search for a Grand Unified Theory, which would integrate gravity with the other fundamental forces. It may be that there is no such theory, or that we are no...