Grammatical inflections such as the English plural noun -s and third
person singular verb -s are acquired at different points in time by young
children. This finding is typically attributed to factors such as relative
semantic salience or the distinction between lexical and functional
categories. In this study input frequency, sentence position, and
duration were examined as possible contributing factors. In both
conversations with and stories aimed at young children, noun plural
inflections were found to be more frequent than third singular verb
inflections, especially in sentence-final position. Analysis of the speech
of four mothers reading stories to their two-year-old children confirmed
that duration differences also exist in the input. Because fricatives were
lengthened in sentence-final position and plural nouns were much more
likely to appear in these positions than were third singular verb forms,
plural nouns were significantly longer than third singular inflections on
average. The possible implications of these findings for language
learnability theories and accounts of grammatical deficits in specific
language impairment are discussed.