OVER-EXTENSION
In language acquisition, the use of a lexical item to refer to a wider range of entities than is normal in adult usage. For example, the word DUCKmight be extended to many more types of bird than the adult concept would admit. Over-extension can involve up to a third of an infant’s early words and is common up to the age of about 2 years 6 months. The child’s reasons for including items within a concept often seem to be based upon similarities of shape; but size, texture, sound and movement are also important defining features. Other over extensions reflect similarity of function or association with the same event (NAP to refer to a blanket).
Some over-extensions involve a whole series of loosely linked common features called by Vygotsky a chain complex. An infant was found to apply the term QUAH (= ‘quack’) to a duck on a pond, then extend it to any liquid including milk in a bottle, to a coin with an eagle on it and from that to all round coin-like objects.
Several reasons have been suggested for over-extension. The child may simply have unformed impressions of the world and thus be unable to recognise similarities between items. Or it may be using an approximate word for a concept for which it does not yet have a term. Or it may be actively engaged in forming concepts: imposing patterns upon its experience and learning by trial and error which items are classed together. One version of this last view suggests that the child is trying to assemble a range of exemplars in order to form a prototype for a particular concept.
The opposite phenomenon, under-extension, also occurs, but rather less frequently. It may be the result of a word being acquired in a way that is too dependent upon context. A child might apply WHITE to snow but not to a blank page or DEEP to a swimming pool but not to a puddle.
See also: Concept formation, Mapping, Over-generalisation
Further reading: Aitchison (2003: Chap. 16); Clark (2001); Kuczaj (1999); Neisser (1987)
