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The current rule states that decennial census records are to be released 72 years after they were first collected. The 72-Year Rule is established in federal law under 92 Stat. 915; Public Law 95-416; which was passed October 5, 1978. Under this law, the 1910 census was release in 1982, 1920 census was released in 1992, the 1930 census was released in 2002, and the 1940 census was released in 2012.
Prior to the passage of the 1978 law, access issues for census records were not always as clearly defined. The information below is quoted from Preliminary Inventory Number 161, Records of the Bureau of the Census, compiled by the National Archives in 1964.
Confidentiality of Census Information
The belief that census information should be confidential developed gradually as census inquiries became more and more detailed and there was fear that disclosure would lead to "a widespread reluctance on the part of the public" to give this information. First the census enumerators, then all "sworn employees" of the Bureau, and finally "any establishment or individual" retaining copies of census reports were required to keep the census information confidential. As early as 1851 the Superintendent of the Census ruled that the "unnecessary exposure of facts relating to individuals" was neither contemplated by the act itself nor justified by the intentions and designs of those who enacted the law. He therefore advised the marshals and assistants "to consider the facts entrusted to them as if obtained exclusively for the Government." The act authorizing the 10th census required the enumerator to take an oath not to disclose "any information in the schedules, lists or statements obtained by me to any person or persons, except to my superior officers”. Section 22 of the act for the 13th census forbade any Census Bureau employee to "publish or communicate any information coming into his possession by reason of his employment.”
These restrictions, however, were still not strong enough to insure that accurate information 'would be given to census takers, and it became necessary to extend and strengthen the principle of confidentiality by further legislation…
Access to Census Information
The progressive restrictions on the disclosure of census information have been paralleled, however, by the growth of public demand for access to such information. Authorization for public access to census schedules or to information in them stems from the legislative requirement that copies of schedules of the first 9 censuses be deposited in such publicly available places as the offices of clerks of courts and from the requirement that abstracts of information of the first 10 censuses be publicly posted. Furthermore, beginning with the 11th census, when only one copy of the schedules was required, the Director of the Census was authorized to furnish to State Governors or municipal officers (upon request and upon payment of the actual cost) some information from the population schedules concerning persons enumerated within their jurisdictions. Finally section 32 of the act for the 13th census, July 2, 1909 (36 Stat. 10), authorized the Director of the Census "in his discretion, to furnish to individuals such data from the population schedules as may be desired for genealogical or other proper purposes." This section was reenacted, moreover, as part of section 33 of the 14th census act, March 3, 1919 (40 Stat. 1301), and as section 18 of the 15th census act, June 18, 1929 (45 Stat. 25). Section 18, which is in full force today, is the authority under which the Census Bureau has furnished evidence of age since the passage of social security legislation.
When the question arose concerning possible conflict between the restrictions imposed by section 11 of the 15th census act and the access permitted by section 18 of the same act, the Chief Legal Adviser of the Census Bureau rendered an opinion to the Director of the Census in a memorandum of April 11, 1940. The memorandum stated that section 11 establishes a "confidential relationship between the individuals furnishing the data and the Bureau," and that section 18 simply permits refurnishing the data to the individuals or to those designated by them. Refurnishing such information. to the individual concerned constitutes the use of the information by the individual and not by the Bureau and does not constitute publication. In requesting that information be refurnished to him, the individual has released the Bureau from the confidential provisions imposed by law.
Consequently access to census information has been granted within the limits imposed by law and agency directive. The Census Bureau ruled in the 1920's that the enumerators' oath for the 10th census made confidential the 1880 and subsequent schedules and that the schedules themselves were not to be examined by the public. The public was to be given information from the schedules, however, if "sworn employees" of the Bureau searched the schedules. After 1940 the restriction on the 1880 schedules was lifted, probably in accord with the so-called "72-year ruling" made by the Director of the Census in a letter of August 26, 1952, to the Archivist of the United States. This letter stated that the census schedules in the National Archives might be made available after a lapse of 72 years from the enumeration date of a decennial census for "legitimate historical, genealogical or other worthwhile research," provided that any information derived from them could not be used to the detriment of any person whose records were involved…
"Frequent demands" for access to census information were made to the Census Office as early as 1850. In the 1890's requests for census information, chiefly for genealogical purposes, became so numerous that in April 1899 the Interior Department's Chief Clerk set up a special file to handle the correspondence. While the genealogical requests continued to grow in the 20th century, they were outnumbered by requests made for other purposes. For instance, in fiscal year 1913 the Pension Bureau detailed 28 clerks to search 1850 and 1860 population schedules to establish the ages of applicants for military pensions. In fiscal year 1918, in order to determine the ages of pensioners and to establish the ages of men who had failed to register for military service, 5,043 searches were made. In fiscal year 1923, when old-age-pension laws were in effect in 18 States, census clerks made 13,242 searches, and 5,965 visitors examined census schedules. In fiscal year 1935, when 30 States had passed old-age-pension laws and the Social Security Act had been passed, there were 5,984 visitors and 21,000 searches, with a backlog of 4,ooo unfilled requests. The Selective Service Act and the outbreak of World War II led to many requests for data on age and place of birth--943,808 for the 18 months from January 1, 1941 to June 30, 1942.
We hope this information is helpful. Best of luck with your research!