The developmental origin of inner speech: Evidence from a mental counting task in 3- to 6-year-old children.

Inner speech is a pivotal cognitive tool, yet its developmental timeline remains controversial due to methodological limitations of existing tasks. Specifically, complex memory demands often mask early competence, and self-report measures rely on introspective skills that young children lack. To address these issues, we introduced a novel "Mental Counting Task," a simple paradigm feasible even for preschoolers, to objectively map the early developmental trajectory of inner speech. Participants, children aged 3 to 6 (N = 58) and adults (N = 25), performed five trials of serial number chanting (1 to 5) in both Overt (chanting aloud) and Inner (chanting silently) Speech conditions at a consistent speed, alongside a self-report inner speech scale. We focused on the stability of counting speed by analyzing the Relative Standard Deviation (RSD), a coefficient of variation that normalizes variability, to evaluate temporal stability across these trials. Results showed that RSD was significantly higher in the Inner Speech condition across all ages, reflecting inherent instability due to the lack of sensory feedback. Notably, RSD in children significantly decreased with age, capturing the gradual stabilization process of inner speech. In contrast, self-report scores showed no correlation with counting performance, underscoring the immature metacognition of preschoolers. Additionally, task completion data indicated that stable inner speech becomes robust around 5 years of age, earlier than previously assumed. These findings demonstrate that RSD in this paradigm serves as a sensitive marker for the maturation of inner speech, providing a new solution for elucidating its developmental origins.