It is a rule by which many genealogists plan their lives.

Once a decade, the US Census Bureau collects the names, addresses, and other details of all people living in the country for a census.

And 72 years after Census Day of the national census, the records containing all this information are being shared with the public, including family historians eager to complete their pedigree charts.

This policy, called the "72-year-old rule," was enshrined in law in 1978 and has become part of the current confidentiality commitment that the Office relies on to convince households to be counted.

Files from the historic 1950 census, the first US census to include baby boomers, will be released for the first time this year on April 1.

But exactly why seven and five decades became the specific time period for keeping census records secret has puzzled researchers, including Jessie Kratz, the National Archives historian responsible for releasing the census records.

"Everybody said, 'You know, 72 years is the shelf life,'" Kratz recalls of the oft-quoted statement she first heard when she started working at the Federal Archives.

But the average life expectancy in the US was closer to 73 when the 72-year rule became federal law, the National Center for Health Statistics reported.