6 5.3 Common Knowledge

You don’t have to cite common knowledge. You might think “common knowledge” refers to things you and your friends already know, but in academic circles, common knowledge is defined differently. Common knowledge means information that you or anyone can easily prove through authoritative sources. It is a fact rather than an opinion or something open to debate.

If a fact can easily be verified in multiple authoritative sources (e.g., encyclopedias, dictionaries, reputable websites, and books), it’s probably considered common knowledge. It is common knowledge whether you happen to know that specific fact or not.

MIT provides the following examples of common knowledge:

• Information that most people know, such as that water freezes at 32 degrees Fahrenheit.
• Information shared by a cultural or national group, such as the names of famous heroes or events in the nation’s history.
• Knowledge shared by members of a certain field, such as the fact that the necessary condition for diffraction of radiation of wavelength from a crystalline solid is given by Bragg’s law.”

Can you imagine having to write “Encyclopaedia Britannica confirms that there are 4 quarts in a gallon“? Looks weird, doesn’t it? Factual common knowledge like this does not have to be cited because there is no “original” source for this information – they are simply facts. Even if you didn’t know that Lincoln was the 16th president, or when and where the world-famous artist Pablo Picasso was born, examples like these are still considered common knowledge because you can easily confirm this information in multiple authoritative sources.

Examples that are not common knowledge

Others’ opinions, information open to debate, research findings (including statistics), and recent discoveries are not common knowledge. It is very unlikely that you’ll find 100% agreement or multiple sources for these types of information. Some typical clues have been bolded in our examples below to help you identify opinions, new research, and ideas open to debate, which must be cited.

• Some biographers of Abraham Lincoln say he suffered from clinical depression.
• The quart measurement might have originated in medieval England as a measurement for beer.
• 60% of art majors believe that Pablo Picasso’s paintings are more interesting than his sculptures.
• In recent studies of Y-chromosomes, geneticists have found that Genghis Khan has approximately 16 million descendants living today

Be aware that some common knowledge may also be contextual. For example, what’s common knowledge among microbiologists may not be common knowledge among lawyers or musicians, and vice versa. But if you have any doubt whether something is or isn’t common knowledge, the best practice is to cite!