Activity theory
Activity theory (AT; ) is an umbrella term for a line of eclectic social sciences theories and research with its roots in the Soviet psychological activity theory pioneered by Lev Vygotsky , Alexei Leont'ev and Sergei Rubinstein. These scholars sought to understand human activities as systemic and socially situated phenomena and to go beyond paradigms of reflexology (the teaching of Vladimir Bekhterev and his followers) and classical conditioning (the teaching of Ivan Pavlov and his school), psychoanalysis and behaviorism. It became one of the major psychological approaches in the former USSR, being widely used in both theoretical and applied psychology, and in education, professional training, ergonomics, social psychology and work psychology.
Social complexity
The American sociologist Talcott Parsons carried on the work of the early founders mentioned above in his early (1937) work on action theory. By 1951, Parsons places these earlier ideas firmly into the realm of formal systems theory in "The Social System". For the next several decades, this synergy between general systems thinking and the further development of social system theories is carried forward by Parson's student, Robert K. Merton, and a long line of others, in discussions of theories of the middle-range and social structure and agency. During part of this same period, from the late 1970s through the early 1990s, discussion ensues in any number of other research areas about the properties of systems in which strong correlation of sub-parts leads to observed behaviors variously described as autopoetic, self-organizing, dynamical, turbulent, and chaotic. All of these are forms of system behavior arising from mathematical complexity. By the early 1990s, the work of social theorists such as Niklas Luhmann began reflecting these themes of complex behavior.