Young preschool children frequently talk out loud to themselves as they play and explore the environment. This self-talk is known as private speech. Vygotsky's socio-cultural theory (1934/1986) maintains that children's private speech is used for self-direction and that this language is the foundation for later complex mental activity. According to Vygotsky, private speech is indicative of early cognitive processing and allows us to hear how children think about their own behavior and select courses of action.
The research literature has shown that private speech follows a predictable course of development in normally developing children and in children identified with serious learning problems (Berk & Spuhl, 1995). According to Vygotsky, private speech develops as children turn social speech toward the self to guide and control their behavior. Research supports Vygotsky's (1934/1986; 1930–1935/1978) original observations of an overall curvilinear developmental trend for overt private speech. That is, overt private speech increases in frequency during the preschool period, peaking around the ages of 4 to 6, and then becomes less common later as it gradually is replaced with more covert forms of self-talk, including whispers, inaudible muttering, and eventually silent inner speech (Berk & Winsler, 1995; Krafft & Berk, 1998; Winsler & Naglieri, 2003).
Research investigations have supported Vygotsky's theory on private speech and its relationship to cognitive development (Berk & Winsler, 1995; Bivens & Berk, 1990; Winsler & Naglieri, 2003).