A recent thread on the Ancestry.com census message board raises the question, Why do we have to wait 72 years for the release of census records? We genealogists would love to get our hands on the 1940 census, but we'll have to wait until 2012.
For historical context, I searched the New York Times database for past stories on the release of census records.
A 1930 article extolled the new "immense fireproof vault" in which that year's census returns would be stored. Even then, census records were kept under tight control: reporters invited to the Census Bureau were not permitted "even one peek at a 1910 volume lying on a repair table."
A census official explained that schedules were considered sealed for fifty or sixty years until they ceased to be personal and became genealogical. [NYT, Mar. 30, 1930]
In 1975, a bill was proposed to "solidify in law an informal agreement reached in 1952 between the Census Bureau and the National Archives that called for the release of census returns after 72 years." The article notes that "The Census Bureau has since tried to repudiate the agreement, but it was upheld by the Justice Department and the 1900 census was recently opened to researchers" [NYT, Nov. 24, 1975].
The same article cites instances when census confidentiality was breached.
During World War I, census reports were used to prosecute men trying to evade the draft on grounds of age. But during World War II, the bureau successfully resisted demands that it produce complete lists of Japanese-Americans on the West Coast for deportation.A later article notes that the Census Bureau did disclose "figures showing where Japanese-Americans tended to live" [NYT, Mar. 31, 1980]. (This despite FDR's 1940 proclamation that "No person can be harmed in any way by furnishing the information required" [NYT, Feb. 11, 1940].) There were concerns in 1980 that illegal immigrants would be identified by the federal government through premature release of census records, and I'm sure Arab-Americans in 2001 were glad that the 2000 returns were not yet available.
The 72-year wait will probably stay on the books because of the competing imperatives of the two government entities involved: the Census Bureau, which takes seriously its promise of confidentiality; and the National Archives, which want to make as much information available as the law allows.