A paper by a research group including Kota Otani (Planned Research Group A01) and Yusuke Moriguchi (Planned Research Group A03) has been published in the international journal Acta Psychologica. Dr. Otani was formerly a doctoral student in Group A03 and is now a researcher in Group A01, in which Dr. Ryuta Kuwamizu also participates.

 This study examined children aged 3 to 6 years using a simple task, "counting from 1 to 5 in your head," to investigate the development of inner speech, which is difficult to observe directly from the outside. The results showed that the time children took to count in their heads became more stable with age, and that from around age 5, children were able to count entirely in their heads without relying on their voice or fingers.

 Inner speech is the language we use in our minds without speaking aloud, and it is thought to be involved in thinking, attention, and self-regulation. By using counting, a familiar everyday activity, this study suggests a way to capture the development of inner speech in young children, who find it difficult to describe their inner speech in words.

 The full text is available as open access. We encourage you to read it.

Paper Information

Authors: Kota Otani, Ryoichi Watanabe, Ryuta Kuwamizu, Nozomi Yamamoto, Yusuke Moriguchi

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

Journal: Acta Psychologica, Volume 270, 107716 (2026)

DOI: https://doi.org/10.1016/j.actpsy.2026.107716

Abstract

 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.

Press Release (in Japanese)

University of Tsukuba

"Counting from 1 to 5 Can Measure 'Words in the Head' in Young Children"