The "iron law of oligarchy" states that all forms of organization, regardless of how democratic they may be at the start, will eventually and inevitably develop oligarchic tendencies, thus making true democracy practically and theoretically impossible, especially in large groups and complex organizations.
Iron law of oligarchy, Wikipedia
Michels’s Iron Law of Oligarchy, CSUN,
Robert Michels, the iron law of oligarchy and dynamic democracy, Constellations, 2020-05-21 (PDF)
Susan Eckstein: Politicos and Priests: The "Iron Law of Oligarchy" and Interorganizational Relations, Comparaive Politics , 1977-07 (JSTOR)
C. W. Cassinelli: The Law of Oligarchy, The American Political Science Review, 1953-09 (JSTOR)
Iron Law of Oligarchy, Sociology Guide,
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