The answers to this question say that you are required by law to answer the American Community Survey (ACS) in the United States.
Are you required to answer truthfully?
I'm only asking about questions that have a definitive answer as I guess there are questions that don't necessarily have a correct answer.
This is addressed in 13 U.S. Code § 221:
(a) Whoever, being over eighteen years of age, refuses or willfully neglects, when requested by the Secretary, or by any other authorized officer or employee of the Department of Commerce or bureau or agency thereof acting under the instructions of the Secretary or authorized officer, to answer, to the best of his knowledge, any of the questions on any schedule submitted to him in connection with any census or survey provided for by subchapters I, II, IV, and V of chapter 5 of this title, applying to himself or to the family to which he belongs or is related, or to the farm or farms of which he or his family is the occupant, shall be fined not more than $100.
(b) Whoever, when answering questions described in subsection (a) of this section, and under the conditions or circumstances described in such subsection, willfully gives any answer that is false, shall be fined not more than $500.
(c) Notwithstanding any other provision of this title, no person shall be compelled to disclose information relative to his religious beliefs or to membership in a religious body.
(Note that Title 13 of the US Code is the section governing censuses, and that the Census Bureau is part of the Department of Commerce.)
Some online sources say that this fine is governed by this section "as changed by Title 18", and that the maximum fine is $5000. I believe this refers to USC 18 §3571, which establishes different maximum fines for felonies, misdemeanors, and infractions. In any event, this does not change the legality of the act.
