Conversation so far:
what percentage of nyc apartments are rent controlled
As of 2017, one percent of apartments in New York City are rent controlled.
what conditions have to be met for rent control to occur in new york city
Rent control happens when a tenant has been living continuously in their apartment since July 1, 1971—yes, 1971—in a building constructed before 1947.
when did rent control first occur in new york city
New York's current rent control program began in 1943.

Question: what caused the program to begin
Self-contained rewrite: what caused new york's current rent control program to begin in 1943