The first empirical studies on interactionism were conducted in the 1960s; however, the concept of interactionism dates back as far as Aristotle (384-322 BC; see Shute, 1973). Kantor (1924, 1926) was one of the first to postulate a person-situation interaction theory of behaviour. The main unit of study is the individual as he or she interacts with various types of situations. Later, Tolman (1935) and Lewin (1935) emphasized the importance of taking both the person and situation factors into account. The interaction of person and situation factors was also characteristic of Murray’s (1938) need-press theory, in which the unit of analysis is the organism-environment interaction.