Large numbers of black Americans in search of economic and social opportunities also arrived in northern cities in waves of internal migration during World War I, World War II, and the 1950s. They often first settled in immigrant neighborhoods, and the terms ghetto and slum came to refer to visible poor black neighborhoods that did not disappear through assimilation. Sociologist Kenneth Clark wrote:
America has contributed to the concept of the ghetto the restriction of persons to a special area and the limiting of their freedom of choice on the basis of skin color. The dark ghetto's invisible walls have been erected by the white society, by those who have power, both to confine those who have no power and to perpetuate their powerlessness.… The objective dimensions of the American urban ghettoes are overcrowded and deteriorated housing, high infant mortality, crime, and disease. The subjective dimensions are resentment, hostility, despair, apathy, self-depreciation, and its ironic companion, compensatory grandiose behavior.
Many social scientists later discarded the ghetto metaphor because it carried misleading expectations that the underclass in the inner city would also disappear automatically.
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