Building language: Mean Length of Utterance, Lexical Growth, and Grammatical Accuracy in Spanish-speaking Monolingual Children

Purpose: This study established age-based reference benchmarks for grammaticality and lexical productivity in monolingual Spanish-speaking children using narrative retell tasks. Developmental changes in Mean Length of Utterance in words (MLUw), Total Number of Words (TNW), Percentage of Grammatical Utterances (PGU), and Moving Average Type-Token Ratio (MATTR) were examined in children aged 3;0–6;11 years. Method: A cross-sectional quantitative design was used with 819 monolingual Spanish-speaking children recruited from mainstream educational settings. Narrative retell tasks elicited spontaneous language samples, from which MLUw, TNW, PGU, and MATTR were derived. Age-based descriptive and percentile analyses were conducted. Results: Results showed age-related increases in lexical productivity and grammatical accuracy. TNW increased from 105 words at age 3 to 180 words at age 6. PGU improved from 75% at age 3 to approximately 88–90% at ages 4–6. MATTR remained relatively stable across age groups, ranging from 0.46 to 0.48. Conclusion: Findings support conceptualising language performance as a developmental continuum and demonstrate the diagnostic utility of percentile-based interpretation. This study provides the first comprehensive reference benchmarks for narrative language measures in monolingual Spanish-speaking children and offers clinically relevant tools for more precise assessment in Spanish-speaking contexts.