Since the publication of Vygotsky's Thought and Language in the United States, a number of North American and European investigators have conducted systematic observations of children's spontaneous private speech, giving substantial support to Vygotsky's major hypotheses - particularly those regarding the social origins of higher psychological functions. However, there still remain many vital questions about the origins, significance, and functions of private speech: How can social and private speech be validly differentiated? What kinds of social interactions promote the use of private...