Supplementary Material for: Exploring Speech Entrainment in English Children with a History of Late Talking: Validation, Group Differences, and Conversational Quality

Introduction: Children with a history of late talking (LT) often experience persistent communication challenges despite vocabulary gains, raising questions about the mechanisms that support or hinder conversational success. Speech entrainment—the alignment of acoustic-prosodic features between interlocutors—facilitates mutual understanding and rapport, yet little is known about how it develops in LT children compared with typically developing (TD) peers. This study examines whether LT children show distinct speech entrainment patterns that may contribute to conversational difficulties. Methods: Conversational speech samples were drawn from the Clinical English Ellis Weismer Corpus in the CHILDES TalkBank repository, involving five-year-old LT and TD children matched for age, nonverbal cognition, and socioeconomic status. Speech turns were annotated using forced alignment and manual verification, and inter-pausal units were segmented to capture child–adult exchanges. Acoustic features representing rhythm, articulation, and phonation were extracted, including envelope modulation spectra, rhythm metrics, long-term average spectra, and mel-frequency cepstral coefficients. Two indices of entrainment—proximity (local similarity across turns) and synchrony (parallel changes across turns)—were computed. Sham conversations were generated via block-randomization to validate entrainment measures. Random forest models identified salient features for classifying real versus sham conversations and LT versus TD groups. Elastic net regression predicted conversational quality ratings from six British native evaluators. Results: Validated entrainment measures reliably distinguished authentic from randomized conversations, confirming their suitability for child data. LT children demonstrated reduced synchrony in articulatory and rhythmic domains, while TD children exhibited robust temporal coordination. Conversely, LT children showed stronger proximity in phonatory features, suggesting reliance on surface-level similarity. Classification models using entrainment features modestly differentiated LT and TD groups. Importantly, higher entrainment levels, particularly in voice and articulatory measures, were associated with better conversational quality, with predictive effects more pronounced for LT children. Conclusion: Findings indicate that LT children adopt compensatory entrainment strategies, emphasizing proximity over synchrony, which may partly sustain conversational engagement despite underlying vulnerabilities. Entrainment measures thus offer a dynamic lens for understanding interactional challenges and hold promise as complementary tools for assessing and supporting communication in children with language delays.