Effects of the growth of breath capacities on mean length of utterances: How maturing production processes influence indices of language development

Measures of “mean length of utterance” (MLU) involving morpheme counts in transcripts are widely applied to speakers of all ages and are generally interpreted as an index of developing grammar. Yet no study has examined how the growth of respiratory capacities influences MLU and numbers of forms in utterances. We review longstanding problems of MLU counts and investigate the effects of growing breath capacities using speech samples and measures of vital capacity (VC) of 50 speakers aged 5 to 27 years. The results show that VC correlates strongly with MLU, which associates with rising numbers of long lexemes. This suggests that, in normal development, the growth of VC offers the possibility of producing increasingly long utterances that can influence lexical diversity. Hence, interpreting MLU and co-varying indices of lexical development requires a consideration of the effects of maturing production processes in a perspective where developing speech and language are seen to intertwine.