Funny thing about Soylent Green .... I read the book that it is (very loosely) based on, called "Make Room! Make Room!" Its whole purpose, summed up in a speech near the end, was to get people to use birth control. The author used population growth statistics to show that by (whatever year the book takes place in), there would be too many people and not enough food or space. Soylent Green was not people in the book! That was made up for the movie. It was made from seaweed.
Anyway, what the author failed to take into account was that farming science and methods would improve over time. So here we are, and the world (at least the first world, as depicted in the book) doesn't look a thing like that.