How could the same person be classified by the US census as black in 1900, mulatto in 1910, and white in 1920? The history of categories used by the US census reflects a country whose identity and self-understanding--particularly its social construction of race--is closely tied to the continuous polling on the composition of its population.

By tracing the evolution of the categories the United States used to count and classify its population from 1790 to 1940, Paul Schor shows that, far from being simply a reflection of society or a mere instrument of power, censuses are actually complex...