Lexical diffusion, as characterized by interword variation in production,
was examined in phonological acquisition. The lexical variables of word
frequency and neighbourhood density were hypothesized to facilitate
sound change to varying degrees. Twelve children with functional
phonological delays, aged 3;0 to 7;4, participated in an alternating
treatments experiment to promote sound change. Independent variables
were crossed to yield all logically possible combinations of high/low
frequency and high/low density in treatment; the dependent measure
was generalization accuracy in production. Results indicated word
frequency was most facilitative in sound change, whereas, dense neighbourhood
structure was least facilitative. The salience of frequency and
avoidance of high density are discussed relative to the type of phonological
change being induced in children's grammars, either phonetic or
phonemic, and to the nature of children's representations. Results are
further interpreted with reference to interactive models of language
processing and optimality theoretic accounts of linguistic structure.