You do not need to cite common knowledge—information your readers could easily find in any number of general sources. For example, the current population of the United States is common knowledge among sociologists and economists; psychologists are familiar with Freud’s theory of the unconscious.

As a rule, when you have seen information repeatedly in your reading, you don’t need to cite it. However, when information has appeared in only one or two sources or when it is controversial, you should cite the source.

What is considered common knowledge varies by discipline and by subject within a discipline. If a topic is new to you and you are not sure what is considered common knowledge or what is controversial, ask someone with expertise. When in doubt, cite the source.