After two decades of abnormally high growth, Nagaland’s population declined during the 2001–2011 period. Nagaland’s population has shrunk in the absence of war, famine, natural calamities, political disturbance, and any significant changes in the state’s socio-economic characteristics. This is unprecedented in the history of independent India. In light of the above, this study examines the reliability of the Census of Nagaland between 1981 and 2011 by testing the internal consistency of Census population estimates. It also tries to validate the Census estimates using information from other sources like Sample Registration System and National Family Health Surveys. The analysis shows that the Census substantially overestimated the population of Nagaland in 1991 and 2001 and raises questions about the Indian state’s institutional capacity to design empirically informed policies.