Permit system entangled in red tape
Every citizen on the mainland is issued with a residence permit, or hukou, that identifies the carrier as 'rural' or 'urban'. Urbanites are registered as residents of a particular urban authority, which entitles them to the social services - education, healthcare, social security - provided by that authority only.
The peculiarly draconian nature of the system, which was designed to restrict mobility, crystallised during the famine of the 1960s, when it was essential to control the number of urban hukou holders, who were entitled to grain rations.
Since the late 1990s, reforms have been introduced to make it easier for qualified rural migrants to register as urban residents, and for urban residents to transfer their hukou between urban authorities.
Local governments now have almost complete control over population management in their jurisdictions, creating a jumble of policies catering to the varied needs of the numerous urban authorities.
Although many of China's huge floating population of rural migrants are eager to swap their rural hukou for an urban one, many farmers are reluctant to give up the user rights to farm land that come with a rural hukou. It makes sense for at least one family member to keep a rural hukou, so the family keeps its safety net - the family plot of land.