It wasn't until 1980 that the government decided to collect Census data by mail-in survey. The shift to a survey had dramatic effects on at least one Census category: race.

Before 1980, Census enumerators categorized people into racial groups based on their appearance. They did not ask respondents how they characterized themselves. Instead, they made a judgment call, drawing on explicit instructions given to the Census takers. On a mail-in survey, however, the individual self-identified. They got to tell the government what race they were instead of letting the government decide. There were at least two striking shifts as a result of this change:

First, it resulted in a dramatic increase in the Native American population. Between 1980 and 2000, the U.S. Native American population magically grew 110%. People who had identified as American Indian had apparently been somewhat invisible to the government.

Second, to the chagrin of the Census Bureau, 80% of Puerto Ricans choose white (only 40% of them had been identified as white in the previous Census). The government wanted to categorize Puerto Ricans as predominantly black, but the Puerto Rican population saw things differently.

I like this story. Switching from enumerators to surveys meant literally shifting our definition of what race is from a matter of appearance to a matter of identity. And it wasn't a strategic or philosophical decision. Instead, the very demographics of the population underwent a fundamental unsettling because of the logistical difficulties in collecting information from a large number of people. Nevertheless, this change would have a profound impact on who we think Americans are, what research about race finds, and how we think about race today.

See also the U.S. Census and the Social Construction of Race and Race and Censuses from Around the World.