Facing the challenge of variation in neuropsychological populations: lessons from biology.

Two ways of dealing with variation in biological populations are discussed. The first is referred to as the natural state model, an approach originated by Aristotle. The second emerged when biologists understood that lawful variation could be ascribed to an entire population of individuals as an organizational unit. This article establishes that the emphasis on single-case studies is driven by assumptions on the nature of variation that conform exactly to the natural state model. By contrast, the alternative case-series approach is consistent with population thinking in modern biology.