It was a decade when tens of millions of people experienced mass unemployment and social upheaval as America clawed its way out of the Great Depression and rumblings of a world war were heard from abroad.

A poster of the 1940 Census.

It’s such a rare gift... especially for people who believe that establishing their family trees is important for understanding their relationship to American democracy, the history of our country and, to a larger sense, of themselves

Now, intimate details of the 132 million people who lived through the 1930s will be disclosed as the US government releases the 1940 census to the public on April 2 for the first time.

After 72 years of privacy protection lapses, access to the records will be free and open to anyone on the internet − but they will not be immediately name-searchable.

For genealogists and family historians, the 1940 census release is the most important disclosure of ancestral secrets in a decade and could shake the branches of many family trees.

Scholars expect the records to help draw a realistic portrait of a transformative decade in American life.

Researchers will be able to follow the movement of refugees from war-torn Europe in the latter half of the 1930s and sketch out in more detail where 100,000 Japanese Americans interned during World War II were living before they were removed.

They will also be able to fully trace the decades-long migration of blacks from the rural South to cities.

Henry Gates, a Harvard University professor and scholar of black history who has promoted the tracing of family ancestry through popular television shows, said the release of the records would be a great contribution to American society.

Prof. Gates, whose new PBS series Finding Your Roots begins on Sunday, said the goldmine of 1940 records would add important layers of detail to an existing collection of opened census records dating to 1790.

“It’s such a rare gift,” he said, “especially for people who believe that establishing their family trees is important for understanding their relationship to American democracy, the history of our country and, to a larger sense, of themselves”.

Margo Anderson, a census historian, said the release of the records could help answer questions about Japanese-Americans interned in camps after the outbreak of war.

“What we’ll be able to do now, which we really couldn’t do, is to take a look at what the Japanese-American community looked like on the eve of evacuation,” added Ms Anderson, a professor of history and urban studies at the University of Wisconsin-Milwaukee.

More than 120,000 enumerators surveyed 132 million people for the Sixteenth Decennial Census − 21 million of whom are alive today in the US and Puerto Rico, according to the US Census Bureau.

The survey contained 34 questions directed at all households, plus 16 supplemental questions asked of five per cent of the population.

New questions reflected the government’s intent on documenting the turbulent decade, by generating data on homelessness, migration, widespread unemployment, irregular salaries and fertility decline.

Some of the most contentious questions focused on personal income and were deemed so sensitive they were placed at the end of the survey.

Less than 300,000 people opted to have their income responses sealed.

In part because of the need to overcome a growing reluctance by the American public to answer questionnaires and fears about some new questions, the bureau launched its biggest promotional campaign up to that time, according to records obtained at the Franklin D. Roosevelt Presidential Library and Museum in Hyde Park, New York.

It opened its first Division of Public Affairs to blanket the country with its message, reaching out to more than 10,000 publications and recruiting public officials, clergy and business owners to promote it.

But some experts say enthusiasm for the release could be dampened by the lack of a name index, especially for novices.

Ancestry.com is also working to make the census records searchable by indexing almost all fields and providing proprietary tools to mine the data.

Josh Hanna, a senior adviser for the company, said the 1940 census would be the biggest database of its kind.