Inner speech, commonly described as the silent conversation individuals have with themselves, plays an essential role in cognitive development. It supports higher mental functions such as attention, working memory, planning, problem-solving, emotional regulation, and self-control. Although adults frequently rely on internal verbalization to organize their thoughts and behaviour, understanding how this ability develops in young children has remained difficult because preschool-aged children often cannot accurately describe their internal mental experiences.1 

To overcome this limitation, investigators from the University of Tsukuba designed a simple behavioural task to assess the development of inner speech in children aged 3–6 years and in adults. Participants were asked to count from 1 to 5 repeatedly under two different conditions: first by counting aloud and then by counting silently while attempting to maintain the same pace during each trial. Researchers measured the consistency of counting speed across repeated attempts and compared performance between overt (spoken) and covert (silent) counting. Participants were also questioned about whether they believed people could use words internally without speaking them aloud, thereby assessing explicit awareness of inner speech2.

The study demonstrated a clear developmental trend. Silent counting became progressively more stable with increasing age, with older preschool children showing significantly less variation in the time required to complete repeated mental counting tasks. In contrast, spoken counting showed minimal age-related differences, suggesting that improvements reflected maturation of internal language processes rather than simple gains in counting ability or numerical knowledge2. 

Another important observation was that the ability to count entirely in one's mind without moving the lips, fingers, or vocal apparatus emerged predominantly among older preschool children. Approximately five years of age appeared to represent a key developmental milestone, as children around this age demonstrated greater consistency in performing silent counting tasks. These findings indicate that inner speech gradually becomes more efficient and stable during the preschool years as children's executive functions continue to mature1,2.

Interestingly, although adults readily recognized that people can think using internal words without vocalizing them, younger children often lacked this explicit understanding. Nevertheless, many children were still capable of successfully completing the silent counting task. This suggests that the practical use of inner speech may develop before children become consciously aware of or are able to describe their internal verbal processes. In other words, children may employ internal language to support cognition long before they can verbalize that such mental activity exists2,3.

The study also highlights the value of a simple silent counting paradigm as a practical tool for investigating cognitive development. Unlike neuroimaging or sophisticated laboratory techniques, this method requires no specialized equipment and can be administered easily in young children. Such an approach may facilitate future research examining the relationship between inner speech, executive functioning, language acquisition, and neurodevelopmental disorders, including attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD), in which internal verbal regulation may differ from typical developmental trajectories3,4.

Overall, these findings provide new evidence that inner speech develops progressively during early childhood and becomes increasingly organized by approximately five years of age. The study further suggests that the capacity to use internal language emerges before children acquire conscious insight into this cognitive process, offering valuable perspectives on the development of self-regulation and higher-order thinking.

References:

1. Vygotsky LS. Thinking and Speech. In: Rieber RW, Carton AS, editors. The Collected Works of L.S. Vygotsky. Vol. 1. New York: Plenum Press; 1987.

2. Kikuchi Y, et al. Development of inner speech assessed using a silent counting task in preschool children. Acta Psychologica. 2026. doi:10.1016/j.actpsy.2026.105327.

3. Alderson-Day B, Fernyhough C. Inner speech: Development, cognitive functions, phenomenology, and neurobiology. Psychological Bulletin. 2015;141(5):931-965. doi:10.1037/bul0000021.

4. Diamond A. Executive functions. Annual Review of Psychology. 2013;64:135-168. doi:10.1146/annurev-psych-113011-143750.