Question

By the 1970s how much of the world's population lived under communist regimes

Answer 1

One-third of the world's population lived under communist governments at that time.

The largest communist nations were the Union of Soviet Socialist Republics (USSR) and the People's Republic of China.  Nations in Eastern Europe were under communism and Soviet influence, behind what Winston Churchill called the "Iron Curtain."  Communism had also spread elsewhere around the globe, in places like Cuba and Vietnam.