Private speech, known colloquially as “talking to oneself,” is used to describe any type of speech not obviously directed toward another person. Broadly classified, human speech falls into one of two categories: social speech, serving the function of social communication; and private speech, frequently serving the function of self-regulation. Research on private speech principally began with the work of Lev Vygotsky (1934/1986; see also Piaget, 1926). According to Vygotsky, the frequency of children's overt private speech rises during early childhood, then declines and virtually disappears. He suggested that its disappearance is the result of the child's progressive differentiation of social and private speech. As the child comes to utilize private speech's self-regulatory function, overt private speech is replaced by internal verbal thought. This internalization of private speech enables the child to participate in more sophisticated, adult forms of cognition.

Current research largely supports the Vygotskian view. Researchers have documented the rise in frequency of overt private speech during the preschool years and its gradual decline during the early elementary school years (Bivens & Berk, 1990; Díaz & Berk, 1992; Winsler, de León, Wallace, Carlton, & Willson-Quayle, 2003). Qualitative changes in private speech production have also been documented. At around age 3, private speech emerges in overt forms; at around age 5, the volume of utterances declines and partially covert forms predominate; eventually, even whispers and mouth movements tend to disappear as private speech becomes fully covert (Díaz & Berk, 1992; Winsler, Carlton, & Barry, 2000).