In formal academic writing, students are expected to provide citations for all information they have obtained from sources except for what might be classified as "common knowledge." Yet what is "common knowledge"?

Common knowledge is information that is presumed to be shared by members of a specific "community" — an institution, a city, a national region, the nation itself, Western vs Eastern civilization, a particular race, ethnic group, religion, academic discipline, professional association, or other such classification.

It is obvious that members of each of these communities could share a "common knowledge" that may not be well-known to outsiders. If one is writing exclusively for members of that group, certain things may not need to be cited that would need to be cited for non-members of the group. The point in these examples is that "common knowledge" is highly relative to the audience and subject of one's writing, as well as the level. The less formal the writing, the less citation and documentation is generally expected as appropriate for the writing.

Formal Academic Writing Stresses Citation per se

However, academic writing is formal by nature. While it may be read mainly by members of a particular field or discipline in which there is an assumed shared "common knowledge," one cannot assume that all readers of academic writing will in fact share the assumed knowledge of the readers the author had in mind while writing. The relative longevity of good academic writing itself will result in the common knowledge of its readership changing as the decades go by, whereas the paper itself will not change.

And it should be evident that, no matter how "common" certain knowledge may be, if an exact quotation or close paraphrasing of this knowledge is taken from a source then it must be cited to the original author and source in order to avoid plagiarism, regardless of whether the idea or fact(s) involved are "commonly known" or not.

If there is any doubt about whether or not to cite, the formal nature of academic writing itself suggests citing. If one must err, it is better to err by being overly certain rather than assuming information to be "commonly known" when in fact it may not be (and the lack of its citation may thus then leave one open to charges of sloppy scholarship or even plagiarism). In short, when in doubt, cite.

However, two areas in which "common knowledge" can generally be assumed are historical dates and facts and established principles within a certain field. Even these are open to interpretation.

Historical Dates and Facts

"Dates and Facts" has a wider interpretation than the words suggest. For example, one would not need to cite a factual statement like "Helsinki is the capital of Finland" regardless of the equal fact that Helsinki has not always been the capital of Finland. Both dates and history are involved in when Helsinki became the capital and what city was capital before Helsinki. But the point of Helsinki presently being the capital, and having been so throughout Finnish independence, can be assumed to be either (a) commonly known by educated individuals even outside of Finland, or (b) a fact that can be easily looked-up even if one does not know, and most importantly, a "fact" which is not subject to contention. By contrast, "Jerusalem is the capital of Israel" is open to considerable contention even if this status is claimed as "fact" by the government of Israel.

On the other hand, a statement like "In the 19th century Helsinki was considered the leading example of Empire-style architecture in all of Northern Europe" would always need citation. This would not be common knowledge even for most Helsinki residents; moreover it isn't an undisputed "fact" as such but rather an "opinion" with which others may disagree. Not only is the central point open to dispute, but so is the definition of "Empire-style architecture" and "Northern Europe" as they apply to the statement. Thus, citation of the source is essential.

Common knowledge is not limited only to what one "knows" without needing to consult references. For example, say you were writing about when Urho Kekkonen resigned the Presidency of Finland and needed to look up the date to make sure it was 1981. Still, the date he resigned could be assumed as common knowledge even if you didn't know it without needing to consult a source. Such knowledge is easy to look up, and the date is not subject to change or dispute; therefore it is "common knowledge."

Likewise, stating that "Abraham Lincoln was the 16th President of the United States" would not require a citation; even if most Americans could not tell you where Lincoln was in the numerical order (not to mention non-Americans, many of whom would not even know a person named Lincoln had been a President). Again, this is knowledge that is easily found, is not changeable, and thus can be assumed to be "common."

Finally, saying that "the population of Finland has been about 5 million for the past several decades" could be considered common knowledge, as it is a general statement that, while not well known by those outside Finland, could be confirmed by looking in any almanac or atlas over the past 30 years. However, "the population of Finland in July 1999 was 4,992,311" is so exact that it must have come from a specific source which must be cited. Such a specific number would almost automatically be open to argument (as to how it was determined, if nothing else), and therefore one needs to cite the source from which it was taken so readers can refer to it for further detail on the census methodology.

Established Principles in a Field

What is considered common knowledge will vary from one field to another. Medical doctors who write research papers do not have to cite their sources for anatomical terminology or common pathological procedures, as such basic knowledge would be considered as common to any doctor reading the paper — whereas it would usually not be common knowledge for a non-doctor.

Similarly, writers in the field of language could assume knowledge of such things as the parts of speech as "common knowledge." It would be naive to cite your source for the definition of a "verb" or an "adjective" unless, of course, you had a definition which was markedly different from how it would "commonly" be defined.

In summary, both careful thinking and a certain amount of common sense must be employed when determining what is "common knowledge" in academic writing. Statistics, changeable data, "facts" open to contention, etc., must always be cited regardless of how "commonly" they may be known. And in general, wherever there is any doubt, it is safer to cite one's source rather than rely on an assumption of common knowledge.