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

Now, intimate details of 132 million people who lived through the 1930s will be disclosed as the US government releases the 1940 census on April 2 to the public 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 more pointillistic portrait of a transformative decade in American life.

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 5 percent 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.

(Read by Emily Cheng. Emily Cheng is a journalist at the China Daily Website.)

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(Agencies)