Census Trivia #9

Since the 1790 census, New York's population has steadily risen or exploded from decade to decade.  Only twice did it ever decrease.  What are those two decades?  (Hint:  In the first decline, the city lost only one hundred thousand residents.  In the second, and much worse, New York lost almost one million.) 

Answer after the jump.

THE 1950's AND THE 1970's