Activity theory was developed out of L. S. Vygotsky's cultural-historical theory by one of his two main collaborators, A. N. Leont'ev, beginning in the late 1930s. It has evolved into a major direction in Russian social psychology and now has adherents worldwide, influencing studies in education, language socialization, computer interface design, and expert work, among others. (It is not to be confused with the classroom Activity Approach of the Deweyan progressives in the United States.)