How do I reference a theory in my essay?

APA:

When referring to a model, theory, or test, you only need to provide a citation the first time the model, theory, or test is mentioned in the text. After that, it is not necessary to keep including the citations when the model, theory, or test is mentioned again. Example:

Cite the first time you mention the theory in the text:

 “Leader–member exchange (LMX) theory (Graen & Uhl-Bien, 1995) is . . . .”

After that, you can just use the phrase:

 “LMX theory” without providing the citations.

Please note when mentioning a theory, philosophy, therapy, or technique, and also disorders, the rule is not to capitalise e.g. cognitive behaviour therapy (CBT). Acronyms are capitalized to show they are acronyms, not because the terms that form them are necessarily capitalized.

Another example of how to cite a theory:

One theory of exceptional employee behaviour posits that star employees “have disproportionately high and prolonged performance, visibility, and relevant social capital” (Call, Nyberg, & Thatcher, 2015, p. 630).

Also refer to the links below from the American Psychological Association APA style blog.

Chicago:

Though usage varies widely, Chicago recommends that names of laws, theories, and the like be lowercased, except for proper names attached to them. For example:

Avogadro’s hypothesis (or Avogadro’s law)

the big bang theory Boyle’s law

(Einstein’s) general theory of relativity

Newton’s first law

Cite the source of the theory according to the resource type in your footnotes. You can then follow the rules for abbreviated/short form footnotes for subsequent citations referring to the same work.