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The schema concept has had a long and varied history in psychology. It was introduced by Bartlett (1932), who observed that subjects' reproductions of a story showed systematic deviations from the original. Elements in the original story that were uncommon or strange were lost or changed and elaborated such that they became more common and made more sense to the subjects, and infrequent words and concepts were replaced by more common ones. Bartlett explained this normalization behavior by assuming that humans adapt incoming information to existing knowledge structures in long-term memory and that they understand new information in terms of these structures. These knowledge structures were called schemas. Schemas may represent our knowledge of stereotypical events such as doing the laundry or cooking a meal (these schema structures are usually called scripts, Schank & Abelson, 1977), of objects and natural categories (see Anderson, 1985, for a discussion), and the knowledge underlying routine behavior (Norman, 1981). They provide a basis for explaining many different phenomena of information processing and memory functioning, such as inferencing, elaborating, stereotyping, reconstruction, false memorization, and the occurrence of slips in task performance. When, for instance, a reader encounters the word forest in a text, he is not surprised to see the noun phrase the trees, with the definite article, in the next sentence even though no trees were explicitly introduced before.
