America in the 1950s was a middle-class society in a way that America in the 1990s is not.
Majority of the people in America during the 1950's belong to the middle-class society. 
mnli hypothesis: Majority of the people in America during the 1950's belong to the middle-class society.  premise: America in the 1950s was a middle-class society in a way that America in the 1990s is not.