Contributing factors leading to the Harlem Renaissance were the Great Migration of African Americans to northern cities, which concentrated ambitious people in places where they could encourage each other, and the First World War, which had created new industrial work opportunities for tens of thousands of people.
Agricultural yields were about 2 tan (a unit of about 110 pounds or 50 kilograms) of grain per mu during the Song dynasty, compared with 1 tan during the early Han and 1.5 tan during the late Tang.
