Approximately 13 million Chinese lack hukou, the formal household registration. This prevents them from claiming full citizenship rights, including social welfare, formal identity documents and employment in the state sector. The government blames birth planning policies for the unregistered population, but this explanation ignores the role of internal migration. Because citizenship rights are locally determined and the hukou system is locally managed, migrants face significant barriers to registering their children. This article systematically analyses the political determinants of the unregistered population nationwide. Based on a logit analysis of a sample of 2.5 million children from the 2000 census, I find that children born in violation of the one-child policy do have lower rates of registration and that children born to migrant mothers are four times more likely to be unregistered than registered. Continuing government focus on the effect of birth planning ignores the more fundamental institutional barriers inherent in the hukou system.