Understanding Children’s Development of American Sign Language Using Innovative Measures of Morpho-syntactic Complexity (ASL MLU) and Diversity (ASL-IPSyn)

This article reports the morpho-syntactic analysis of over 80 hr of longitudinal production data from four deaf children aged 1;05 to 4;02, acquiring American Sign Language (ASL) from their deaf, signing parents. We analyzed monthly sessions for the Number of Different Words per minute (NDW), Mean Length of Utterance (MLU) in both words and morphemes, and also applied the American Sign Language Index of Productive Syntax (ASL-IPSyn), a novel ASL adaptation of the English measure originally developed by Scarborough (1990). The ASL-IPSyn is based on ASL-specific grammatical components across five subscales: Noun Phrases, Verb Phrases, Questions/Negation, Sentence Structures, and Depiction. Results show that NDW, MLU, and ASL-IPSyn scores all correlate with age, indicating successful measurement of age-related growth. In addition, although MLU and the ASL-IPSyn measure different constructs, they are significantly correlated with each other and with NDW. However, while MLU scores increase over a relatively modest range, the scores on the ASL-IPSyn are more dispersed, making it more useful for examining growth over time. Finally, we used the data produced by the ASL-IPSyn to characterize the approximate age of acquisition for individual grammatical structures. These ages are in general alignment with previous reports (where available), demonstrating the external validity of the measure. We conclude that all three of the measures described in this article are useful and replicable tools for examining ASL development among toddler-to-preschool-aged children.