In Article 1, Section 2, the Constitution includes the phrase:
[An] 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.
Congress first met in 1789, and the first national census was held in 1790.
There was actually some debate about whether, how, and on what timetable a census should have been held. In early 1790, several members of Congress argued against a census prior to the next election. Some in the Congress, who advocated an immediate census, noted that those who did not want one were the people from states which were generally regarded as being over-represented in the Congress based on the initial figures provided for in the Constitution. Others were concerned about the questions to be asked in the census, while others felt that more questions should be asked to get a better picture of the citizenry.
For example, on February 2, 1790, Samuel Livermore of New Hampshire lamented that the proposed question about profession would be hard for his constituents to answer, since some had three or four professions, depending on the season. Connecticut Representative Theodore Sedgwick, on the same day, wondered why the questions were not extended further — "The state of society could be ascertained, perhaps, in some degree, by observing [the] proportions."
The final bill, Statute 2 of March 1, 1790, provided that census marshals and assistants be appointed. The marshals were directed to:
cause the number of the inhabitants within their respective districts to be taken; omitting in such enumeration Indians not taxed, and distinguishing free persons, including those bound to service for a term of years, from all others; distinguishing also the sexes and colours of free persons, and the free males of sixteen years and upwards from those under that age.
The act directed that the names of the heads of families be recorded, the number of white males sixteen and older, the number of white males under sixteen, the number of white females, the number of all other free persons, and the number of slaves. Failure of an assistant marshal to make a return, or to make a false return, was punishable by a $200 fine. Failure of a marshal to do the same was punishable by a fine of up to $800. The questions about profession, and other information Representative Sedgwick spoke of, were not made part of the final census. Census day was set at the first Monday in August, 1790. Failure to cooperate with a marshal or assistant was punishable by a $20 fine.
Today, the controlling law for the U.S. Census is Title 13 of the U.S. Code That law requires that the census be conducted on or about April 1, 1980, and every ten years after that. The returns must be made available within nine months in order to apportion members of the House of Representatives to each of the states. In the intervening years the law requires the Census Bureau to gather statistics about the residents of the United States for use by Congress. The decennial census is provided for at 13 USC 141.
The law states that the count done in 1980 and every ten years thereafter shall be an actual headcount. The count done in the intervening years need not be an actual headcount, but may use statistical sampling methods to get a reasonable approximations of a head count.
There are fines for non-response and for false response as well,
though the amount has risen from the 1790's $20. Today failure to respond can result in a $100 fine; providing false answers is a more severe offense, and carries a $500 fine. Recent news reports, however, indicate that punishment for failure to respond is not usually enforced. The controlling section of the Code is 13 USC 221.
Today, all persons are counted as whole persons — the original census counted "other persons" (slaves) as three-fifths persons for the purposes of apportionment. This fractionalization was removed by the 14th Amendment. The Attorney General ruled, in 1940, that there were no longer any Indians in the United States who could be classified as "not taxed." In the Constitution, non-taxed Indians are not counted.
The number of questions in the decennial census has varied widely since the first in 1790, where census takers logged the name, gender, and race or each member of a household, to 2000, where a multi-page form with dozens of questions was sent to one out of every six households. In 2010, the Census Bureau trimmed the questionnaire to just the basics: name, gender, race, and ethnicity or each person, and whether the dwelling was owned, rented, or "occupied without payment of rent."
A more detailed list of questions, called the American Community Survey (ACS), is sent to selected households in addition to the shorter headcount forms and in non-decennial years to allow the Bureau to do statistical sampling. According to the Census Bureau, about 3 million households are selected to receive the ACS each year.
The law requires, in the case of both the decennial census and the ACS, that all households that get a form must fill it out in its entirety, under penalty.
Generally speaking, the Census Bureau is not interested in levying the fine, and prefers to gather the data. If a survey is not returned, the Census can follow up by phone or with a personal visit. There is, however, the threat of a penalty for non-response. The current penalty is $100 for failure to fill out the census forms.
The authority of the Congress to conduct the census in whatever way it wishes, and thus to require that the forms be filled out is found in the Constitution itself, which notes:
[The Census] shall be made ... in such Manner as [Congress] shall by Law direct.
The Congress is also authorized to ask various questions in the census aside from the basic headcount by virtue of this clause and by virtue of the Necessary and Proper Clause.
Advice to leave the form blank or to fail to fill it in may actually bring more of the government into your life than you want — as noted above, unfiled and incomplete forms will be followed-up upon by actual census workers, either in person or by telephone.