 He started his career in the Indian Statistical Institute and worked on the Anthropological Survey of India before joining the Department of Anthropology at Columbia University in New York in 1966; he was a lecturer and later an adjunct professor and headed the social demography section in the International Institute for the Study of Human Reproduction.He was also a senior associate in the Population Council in New York and a patron and vice president of the Elmhirst Institute of Community Studies at Santiniketan, and served as chair of the population commission in the International Union of Anthropological and Ethnological Sciences.Nag was a pioneer of demographic anthropology.