Those ethnic groups which reside in the jurisdiction of recognized nations rather than having an independent state of their own. Fourth-world peoples are excluded from positions of power in those governments, and frequently have de facto if not de jure status as second-class citizens or worse.
The Palestinians, Kurds, and Roma are all examples of members of the Fourth World.
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An overview description for a series of interconnected comic books drawn and written by artist Jack Kirby, after leaving Marvel Comics in the 1970's due to his frustration with Marvel management; the books dealt with science fiction and mythological themes; the books were entitled, THE NEW GODS, FOREVER PEOPLE, and MISTER MIRACLE. None of the characters could be described as the sort of conventional 'superheroes' that Kirby and others had created in the past, but the books revolved around the idea of what could be called 'demi-gods', some good, others evil, but technologically and physically advanced over humans - the books dealt with the interaction between the 'good' gods of NEW GENESIS and the 'evil' ones of APOKOLIPS and their eventual contact with earth.
You will find opinions among Jack Kirby fans equally divided about the Fourth World books he did; but the themes are found in works like: Fantastic Four, Chariots of the Gods, The Day The Earth Stood Still, Childhoods' End, More Than Human, Eternals, Forbidden Planet, Dr.Who? .. and too many more to list ..