The United States Constitution requires the Census be taken every ten years as a means of apportioning direct taxes and to determine the number of congressional representatives in the House for each state. Originally, the census would count all free peoples, indentured servants, Native Americans who paid taxes, and slaves who only counted as three-fifths of a person.
“Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of Free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons. The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct” (Constitution, Article 1, Section 2).
Since the first census was taken in 1790, there have been 22 censuses recorded (each 10 years apart). The census has a 72-year restriction limiting access to the 1950-2010 records to individuals and heirs. This means that for general research purposes, the 1940 census is the most recent open for general use. Up until the 1890 census, US Marshals performed the census, after which specially trained census-takers took over the task. The Census Bureau provides the instructions and questionnaires used for each census.
From 1850-1940 you can find the following details for all individuals recorded in the census
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