Western study of private speech has been mostly directed at looking for empirical data to test L. S. Vygotsky's hypotheses (Vygotsky, 1934/1987). The great majority of empirical studies have been focused on three main aspects of private speech: (a) its origin and evolution; (b) its cognitive function and relations with task performance; and (c) the structure of private speech productions (Berk, 1992; Díaz, 1992; Fuson, 1979; Krafft & Berk, 1998; Montero, de Dios, & Huertas, 2006; Winsler, 2006; Winsler, de León, Carlton, & Willson-Quayle, 2003; Winsler, Díaz, & Montero, 1997; Zivin, 1979).
The studies carried out on the origin, development, and function of private speech reflect the polemic maintained by J. Piaget and L. S. Vygotsky on its nature in the 1930s. For Piaget, egocentric speech is a subproduct of the general cognitive egocentrism that governs children's mental life during early childhood (3–4 years to 6–7 years). Egocentric speech lacks specific cognitive functionality and, therefore, is indicative of child cognitive immaturity. From the Piagetian perspective, it has to be expected that as the child is socialized and egocentrism decreases, the number of private speech productions will be reduced, until they disappear. On the contrary, Vygotsky's hypothesis on the internalization of language clearly contrasts with the ideas of Piaget, at the same time that it predicts a distinct relationship between private speech and age. According to Vygotsky, private speech has its origins in social speech, which regulates the child's first social interactions (Vygotsky, 1934/1987).