America in the 1950s was a middle-class society in a way that America in the 1990s is not.
Not much has changed between the periods of 1950 to 1990 in America. 
mnli hypothesis: Not much has changed between the periods of 1950 to 1990 in America.  premise: America in the 1950s was a middle-class society in a way that America in the 1990s is not.