I often see libertarians contrasting (as Walter Block does here, for example) those census questions that the Constitution makes it legally mandatory to answer versus those it doesnt.
But as Ive argued elsewhere:
Although as an anarchist I dont especially care what the Constitution says one way or another, its worth noting that all the Constitution authorizes is the mere conducting of a census; it doesnt authorize mandating compliance, since forcing people to answer is not essential to the conducting of a census. (Private groups conduct surveys all the time without enjoying such power.) So as I read the Constitution (though of course the government doesnt give a damn about how I read it), even if one accepts the Constitutions authority there is still no legally binding obligation to answer any of the questions.
Now it might be objected that the Constitution also grants Congress the power to make all laws which shall be necessary and proper for carrying into execution its powers and the power to compel compliance might thereby be taken to be necessary to the end of enumerating the citizens.
But even if it is necessary, whats authorized is legislation thats necessary and proper; and compelling compliance with the census is not proper.
Why should we accept Spooners rules? I sing, I dance.