TO explain why people's behavior is adapted to their circumstances, one might argue that people who don't adapt don't survive. Selection of the best, rather than rational choice, is what ensures a tight fit between behavior and the demands of the environment. These two mechanisms differ in a number of ways.

Rational choice is concerned with the intended outcomes of action. Selection mechanisms operate through actual outcomes. In explanations of animal behavior, where intentions have at best a minimal place, actual outcomes must bear most of the explanatory burden. It is more controversial which mechanism is the most important in the study of human action.

As explained in chapter III, rationality is no guarantee of success. Faced with a set of symptoms, doctors are acting rationally when they use what they have learned in medical school to diagnose and treat their patients. If a disease is a novel one, they can be quite wrong. Their frustrated patients may well turn to quacks, who choose their treatments more or less at random. One of them may accidentally hit upon a treatment that works. Eventually all patients will receive that treatment, either because the lucky quack takes over the market or because others start imitating him. In either case, the success of the treatment owes nothing to rationality, and everything to accident and selection.