What is the serial comma in APA Style?
The serial comma refers to the comma that appears before the last item in a list of three or more items. According to the American Psychological Association (2010), authors should provide a comma, "between elements (including before and and or) in a series of three or more items:
the height, width, or depth
in a study by Stacy, Newcomb, and Bentler (1991)
in a study by Stacy, Newcomb and Bentler (1991)" (p. 88).
American Psychological Association. (2010). Publication manual of the American Psychological Association (6th ed.). Washington, DC: Author.