The history of scientific disciplines is marked by one or more theoretical transformations that were made possible by the introduction of novel sources of evidence. Physics provides the best examples. The invention of apparatus that permitted measurements of the varied energy profiles emitted by different forms of matter motivated the creation of quantum mechanics and new conception of both the infinitesimal space of the atom and the near infinite expanse of the cosmos. The invention of the amplifier, electrode, and chemical stains for neurons provided surprising information about the brain that required concepts very different from those used by nineteenth-century scholars.

Developmental psychology and the psychology of personality have not often enjoyed the extraordinary advantages of new sources of evidence. The primary data in most studies of the development of personality and pathology remain verbal replies, spoken in interviews or written on questionnaires, that rely on the nontechnical language of everyday conversation. The use of physiological data, direct behavioral observations, or performance on specialized laboratory procedures is relatively rare in developmental studies. Even in psychiatric nosology, referential meanings of diagnostic categories are usually restricted to phenomenological information. The concept social phobia, for example, means that a patient has told a clinician that he experiences severe discomfort when he has to meet new people, avoids such social situations as much as possible, and, in addition, is unhappy with this aspect of his life.