Mean Length of Utterance (MLU) has been widely used to measure children’s early language development in a variety of languages. This study investigates the utility of MLU to measure language development in four agglutinative and morphologically complex Southern Bantu languages. Using a variant of MLU, MLU3, based on the three longest sentences children produced, we analysed the utterances of 448 toddlers (16-32 months) collected using the MacArthur-Bates Communicative Development Inventory, a parent-report tool. MLU3, measured in words (MLU3-w) and morphemes (MLU3-m), significantly correlated with age and other indices of language growth (e.g., grammar and vocabulary). MLU3 measures also accounted for significant variance in language development particular morphosyntactic development. Our results suggest that MLU3-m is a more sensitive measure than MLU3-w. We conclude that MLU measured in morphemes provides a useful addition to other indices of language development in these kinds of morphologically complex languages.

平均话语长度（MLU）已被广泛用于衡量各种语言的儿童早期语言发展。本研究探讨了 MLU 在衡量四种粘着且形态复杂的南部班图语言的语言发展方面的效用。我们使用 MLU 的变体 MLU3，根据儿童说出的三个最长句子，分析了使用家长报告工具麦克阿瑟-贝茨交际发展量表收集的 448 名幼儿（16-32 个月）的话语。 MLU3，以单词（MLU3-w）和语素（MLU3-m）衡量，与年龄和其他语言发展指数（例如语法和词汇）显着相关。 MLU3 测量还解释了语言发展（特别是形态句法发展）的显着差异。我们的结果表明 MLU3-m 是比 MLU3-w 更敏感的测量方法。我们得出的结论是，以语素测量的 MLU 为这些形态复杂的语言中的语言发展的其他指标提供了有用的补充。