The population of China doubled in size during the 9th, 10th and 11th centuries. This growth was made possible by expanded rice cultivation in central and southern Song, the use of early-ripening rice from south-east and southern Asia, and the production of widespread food surpluses. The Northern Song census recorded 20 million households, double of the Han and Tang dynasties. It is estimated that the Northern Song had a population of 90 million people, and 200 million by the time of the Ming dynasty. This dramatic increase of population fomented an economic revolution in pre-modern China. The expansion of the population, growth of cities, and the emergence of a national economy led to the gradual withdrawal of the central government from direct involvement in economic affairs. The lower gentry assumed a larger role in grassroots administration and local affairs. Appointed officials in county and provincial centers relied upon the scholarly gentry for their services, sponsorship, and local supervision.