The reasons why the northern nationalities moved to the Central Plains and the Han nationality moved to the south of the Yangtze River.

The flow of "population migration" in ancient China can be said to be two contradictory trends: first, the population in the border areas with low productivity migrated to the Central Plains with relatively developed productivity; On the other hand, the population in the Central Plains with developed productivity flows to the border areas with relatively backward productivity. Its tributaries can also be said to be two contradictory trends: one is the outflow of Han people; The other is the influx of foreigners. The population in the border areas with low productivity level migrated to the Central Plains with relatively developed productivity level. This kind of population mainly migrated from sparsely populated desert areas to densely populated Yellow River Basin, and the main body of migration was ethnic minorities. Part of this population migration is government behavior. For example, people from Dongyue and Fujian and Vietnam moved in twice in the middle of the Western Han Dynasty, once in the third year of the founding of the founding father of Emperor Wu of Han Dynasty and once in the first year of Yuanfeng. Hundreds of thousands of people moved in, but most of them were ethnic minorities who spontaneously moved into the Central Plains to strengthen the process of their sinicization and feudalism. In the early years of Emperor Han and Emperor Di, the southern Xiongnu "had 34,000 households and 230,000 people" and gradually moved eastward and southward, living in Shaanxi and northwestern Shanxi in Hetao, where the original Han people lived together. At the end of the Eastern Han Dynasty, the five lakes moved inland, with hundreds of thousands of Xiongnu and Xianbei, and more than 500 thousand clan and Qiang people. They moved to Gansu, Shanxi, Hubei, Liaoning and other places. From the Five Dynasties to the Song Dynasty, Jurchen and Mongols moved south, and in the Ming Dynasty, Wala and Manchu also moved south, living together with the Han nationality. As for the reasons why the Han nationality in the Central Plains moved south, there are two reasons: first, the economic development in the south of the Yangtze River, the economic center of gravity moved south, and people moved south for their lives; Second, after the Tang Dynasty, there was a war in the Central Plains, and people were displaced, so they had to leave their homes and go south to escape the war. After the establishment of the Southern Song Dynasty, many gentry also moved south. As a result, the Han people in the Central Plains moved south. These Central Plains Han people are what we call Hakkas today.