 Self-determination theory SDT is a macro-theory of human motivation and personality that concerns people's inherent growth tendencies and innate psychological needs. It is concerned with the motivation behind choices people make without external influence and interference. SDT focuses on the degree to which an individual's behavior is self-motivated and self-determined. In the 1970s, research on SDT evolved from studies comparing the intrinsic and extrinsic motives, and from growing understanding of the dominant role intrinsic motivation played in an individual's behavior but it was not until the mid-1980s that SDT was formally introduced and accepted as a sound empirical theory. Research applying SDT to different areas in social psychology has increased considerably since the 2000s. Key studies that led to emergence of SDT included research on intrinsic motivation. Intrinsic motivation refers to initiating an activity for its own sake because it is interesting and satisfying in itself, as opposed to doing an activity to obtain an external goal extrinsic motivation. Different types of motivations have been described based on the degree they have been internalized. Internalization refers to the active attempt to transform an extrinsic motive into personality-endorsed values and thus assimilate behavioral regulations that were originally external. Edward L. Desi and Richard Ryan later expanded on the early work differentiating between intrinsic and extrinsic motivation and proposed three main intrinsic needs involved in self-determination. According to Desi and Ryan, the three psychological needs motivate the self-to-initiate behavior and specify nutriments that are essential for psychological health and well-being of an individual. These needs are said to be universal, innate and psychological and include the need for competence, autonomy and relatedness.