When you are citing a scientific or academic theory in American Psychological Association, or APA, style, the theory is noted based on the source in which it appears. If you are quoting or paraphrasing a theory in your paper, note whether the source you are referencing is written by the theory's author or by a secondary author commenting on the theory.
Citing a Theory as Written by the Author
Whenever you reference a theory in your paper, include a description of the source in which it appears in your reference list. The basic APA format for citing a book source is as follows:
Author Last Name, First Initial(s). (Year). Book title: Subtitle if applicable. Publisher Year, Publisher Location.
Eliade, Mircea. (1987). The sacred and the profane: The nature of religion. Orlando, FL: Harcourt.
If you reference or quote a theory in the body of your work, include an in-text citation. This notes the author and year of the source in which the theory appears and the page number if a direct quote. For example:
The sacred, as located in relation to the notion of myth and ritual, is defined as "the opposite of the profane" (Eliade, 1987, p. 10).
Citing a Theory in a Secondary Source
When you are citing a scientist's or academic's theory as described in a work by another author, only the information for that work appears in your reference list. There is no need to note that you are using it as a secondary source.
When you reference the theory in the body of your text, the in-text citation should note the source from which the theory was described. For example:
Eliade's theory of the sacred and the profane is identified as a "distinction between mundane and mythic time" (as cited in Conuel, 2000, p. 25).