"Everything in an encyclopedia is Common Knowledge" - actually true? One "rule of thumb" that is often given out by writing centers and tutors (for example, see the Yale Center for Teaching and Learning) is that anything in a general-reference print encyclopedia (e.g. World Book, Encyclopaedia Britannica, etc.) is "Common Knowledge" and never (or, at least, hardly ever) needs a citation in academic writing as facts do not get added to encyclopedias until they become generally recognized in the scientific community as true. The major exception to this is that one must cite if one quotes the exact text of an encyclopedia.

The Yale link above explains that Common Knowledge "...is knowledge that most educated people know or can find out easily in an encyclopedia or dictionary."

To what extent is this actually true?


Is this mostly true with a few exceptions?
Is it partially true and partially false?
Is it almost entirely false?


To be clear, I am asking about general principles and best practices in citation, not any specific context. Contact your editor or instructor.

The statement can be true even if there are minor exceptions. For example, "This is true, except that orbital periods of celestial bodies must always be cited because SCIENCE!!!11!111!one, even if they were first measured hundreds of years ago and verified hundreds of times to within 0.1% of their accepted value today" or "This is really true 99.9% of the time, but there was that old case where an editor of the Encyclopaedia Britannica 1990 had a momentary lapse of professional judgment and allowed a controversial statement on the mating habits of Bactrian camels to be printed despite the fact that it had been first published in 1988 and lacked consensus in the scientific community at that time" (I'm not saying that this actually happened, it's just a hypothetical example).

This question is about general knowledge that constitutes Common Knowledge across fields. There is a concept of field-specific Common Knowledge, e.g. where Boyle's Law is considered common knowledge in Chemistry and Physics and needs no citation but might require a citation if used, for example, in a Gender Studies paper. This question is not about such domain-specific Common Knowledge.

In response to comments, I am using the term "Common Knowledge" in its sense as a term of art in Academia for knowledge that need not be cited because it is sufficiently old, sufficiently well-reproduced, and/or sufficiently non-controversial that a citation would serve little, if any, practical purpose, not the sense of knowledge that most people on the street have memorized. For example, the fact that the typical habitat for the Venus' Flytrap consists of subtropical marshes of the US East Coast is Common Knowledge - the average person you meet on the subway or at the bar might or might not know this, but any decent encyclopedia will explain it to you, and you would be hard-pressed to find a botanist (or any scientist, for that matter) who would seriously challenge it.
Taking the statement “anything in an encyclopedia is common knowledge” at face value reveals it is obviously false. In the comments it was pointed out that half of Americans don’t know what century their civil war was in, but there’s also the fact that two thirds of people worldwide have either not heard of the holocaust or engage in holocaust denial. I recently was in a regional trivia competition with thousands of contestants where I had to explain to the organizers that one of their questions were wrong because they had confused density and mass. Virtually all of the questions on Jeopardy! are answerable if you knew everything in an encyclopedia, but even the specially trained contestants don’t know all the answers. Based on these examples alone, I think the idea that the majority of people know all the facts in most encyclopedias is nonsense.

I would strongly advocate for the idea that encyclopedias are irrelevant to the question of if something should be cited. Besides the fact that the vast majority of people don’t know most of the facts in an encyclopedia, there are other reasons to cite things, including:


Giving credit to the discoverer
Providing a link between your work and previous work on the subject
Making it easier for readers to find similar relevant articles.
Providing accountability for your positions. Sometimes encyclopediae are wrong, and the process of looking for a source would hopefully reveal your mistake.


Even if we pretended that it was common knowledge that every triangle-free graph contains an independent set of size Ω(sqrt(n log n)), all the above reasons to cite that fact would apply.