This is a rule many genealogists base their lives on.

The U.S. Census Bureau gathers information about every person in the country once a decade for a head count.

All records containing all that information, 72 years after the Census Day of a national tally, are made available to the public. This includes family historians who want to complete their genealogy charts.

This policy, known as the “72-Year Rule”, was adopted into law in 1978. It is now part of the current promise to confidentiality that the bureau uses to persuade households get counted.

Files from the 1950 census, the first U.S. census to include baby boomers, will be released this year for the first-ever time.

Researchers have been trying to figure out why census records were kept secret for seven and a half decades. Jessie Kratz is the historian at the National Archives. She is responsible for making census records available.

Kratz recalls the common explanation that she heard when she started working at the federal government’s archives: “Everyone just said, “You know, 72 is a life expectancy,”

The average American life expectancy was closer to 72 years at the time that the 72-Year Rule became federally law, according to the National Center for Health Statistics. This could be due to bureaucratic chance.

NPR was asked by the Census Bureau to interview them about the origins of the 72-year-olds. The webpage provided no clear answers. However, the website highlights a 1952 exchange in letters on which the 1978 law is built.

The letters were sent between Wayne Grover (archivist at the time) and Roy Peel, former Director of Census Bureau. They specify that the National Archives and Records Service can disclose any information in these records after “seventy two years” from the date of an enumeration of a census to be used in legitimate historical, genealogical, or other worthwhile research.

Kratz and long-time census watchers believe the letters are part a scattered trail of clues that support their theory about what is behind 72 years of bureaucratic happenstance.

This theory dates back to the beginning days of the National Archives. Kratz explains that the agency had to convince other agencies that it was the right repository for federal records.

The Census Bureau wasn’t convinced in 1934, when the National Archives was founded. It already had its own system to access the Washington, D.C. information.

“These records, which can not be replaced if lost, destroyed, or stolen, would be of little use to the public if they were transferred to the Bureau of Archives” wrote William Lane Austin, Census Bureau Director, in a 1934 memo addressed to the commerce secretary.

A decade later, during World War II, and the same year that the bureau moved from Washington’s downtown to suburban Suitland in Maryland — officials at the bureau had changed their minds. Officials at the bureau had apparently changed their minds.

“Census records up to the year 1870 aren’t confidential and may be used by legitimate research purposes, private individuals,” said a National Archives press release about the transfer of the first batch of census records.

“The National Archives is like the Census Bureau but it is under pressure to limit its research on these records due to war-related demands such as those involving birth information needed for enlistments, or workers in war industries.

1942 was 72 years after 1870.

Scientists are still trying to figure out where the 72-year-olds came from

Kratz, National Archives historian, believes that officials used the 72-year time period as a guideline for when information could become public as part of the 1952 Agreement between the Bureau and Archives over regular transfer of census records.

She says, “That would have been my biggest guess, even though I haven’t seen a piece on paper that says so,” about why the bureau director suggested “the lapse in seventy-two year” in 1952. (When, according to the U.S. surgeon General at the time, the average life expectancy was only 68 years).

Kratz wrote recently a blog post about a dispute that arose in 1970s over the length of census records to be kept secret and who could access them. The 1900 census records were not released until 1973 due to this disagreement. Congress finally settled the matter with its 1978 law.

However, Archivist James Rhoads stated that there was no evidence from the National Archives to explain why 72 years were chosen for the 1952 agreement during a 1973 congressional hearing.

James O’Neill, who was acting as archivist at that time, supported Kratz’s theory at a hearing in 1975, more than two years later.

“The 1870 census records were made accessible when they were transferred from the Archives to the Archives in 1942, 72 year after the census was taken. O’Neill stated that this established the 72-year precedent of restrictions on population census records.

A 77-year rule is possible.

David McMillen is a former Census Bureau employee and later worked at the National Archives. He has attempted to track down this history. The question of “Why 72 years?” remains unanswered.

“It’s funny, isn’t? McMillen says. McMillen says, “There is no definitive answer to this question.”

Margo Anderson, the U.S. Census historian and author of The American Census: A Social History notes that this is a common problem when looking into the records of the once-a decade counts.

Anderson says that “the people who did it last time may no longer be around”. Anderson has also studied the complex history of the removal of privacy protections during World War II, which allowed census records to be used to identify Japanese-Americans living in Washington, D.C.

Today’s clearest conclusion is that the 1978 law allows the 72-Year rule to be renegotiated by the Census Bureau director, and the Archivist of the United States.

There are no indications of active negotiations.

If a person’s life expectancy is what the government wants, then the most recent data on the average U.S. life expectancy could be used to support a new rule regarding how long census records should be protected: 77 years.