Psychology is a fascinating discipline because of the diversity and richness of its subject matter—how people function at multiple levels of analysis, ranging from the brain to society. Psychology is also a frustrating discipline, and for the same reason. The diversity and richness of human experience poses a formidable challenge for identifying common principles with which a comprehensive theory can be generated. Yet the quest for unification is basic to science, one that has met with varying degrees of success in other disciplines. Psychologists are aware of this obstacle to achieving status as...