Proposed initially in 1992 by social psychology researchers, Jim Sidanius, Erik Devereux, and Felicia Pratto, the theory begins with the observation that human social groups consist of distinctly different group-based social hierarchies in societies capable of producing economic surpluses. These hierarchies have a trimorphic (3-form) structure, a description which was simplified from the 4 part biosocial structure identified by van den Berghe (1978). The hierarchies are based on (1) age (i.e., adults have more power and higher status than children), (2) gender (i.e., men have more power and higher status than women), and (3) arbitrary-set, which are group-based hierarchies that are culturally defined and do not necessarily exist in all societies. Such arbitrariness can select on ethnicity (e.g., in the US, Bosnia, Asia, Rwanda, etc.), religion (Sunni versus Shia Islam), nationality, or any other socially constructed category. Social hierarchy is seen not just as a universal human feature- SDT argues there is substantial evidence it is shared, including the theorized trimorphic structure- both among all hominoids as well as other primates.