Developmental relations between thought, language, and behavior have proved to be perennially interesting to psychologists, cognitive scientists, and philosophers (Nelson, 1996; Pinker, 1994; Vygotsky, 1934/1987). To what extent is language separate from thinking? How does language development influence cognitive development? To what extent is language development dependent upon cognitive growth? How is language used by children as a tool for guiding one's thinking, behavior, or problem solving?
One phenomenon that falls at the intersection of many such discussions is children's private speech – children's overt and sometimes partially covert (whispered) self-talk while they are working on something or playing. Children's private speech provides an empirical window for exploring many interesting questions about mind, behavior, and language, especially those having to do with language serving a role in the development of children's executive function or self-regulation. Private speech is typically defined as overt, audible speech that is not addressed to another person (Winsler, Fernyhough, McClaren, & Way, 2004). Inner speech, on the other hand, refers to fully internal, silent verbal thought – that is, speech fully inside one's head.
Research on children's private speech, largely that which originated from within the Vygotskian theoretical tradition, has been summarized and reviewed before on two occasions – first, in Zivin's (1979a) volume entitled The Development of Self-Regulation Through Private Speech (Zivin, 1979b), and then 13 years later in Díaz and Berk's (1992) volume, entitled Private Speech: From Social Interaction to Self-Regulation (Berk, 1992). Since then, however, research on private speech and self-talk has blossomed.