" A citation with multiple authors should use “et al.”
For citations with only two authors, you should always include both authors’ names.
For citations with three to five authors:
- The first use of a citation should include all authors’ surnames – e.g., Smith, Williams, and
- The second and all subsequent uses should name the first author then use “et al.” instead of
the other authors’ surnames – e.g., Smith et al. (1987)
For citations with six or more authors, cite only the first author followed by “et al.” and the year for the
first and subsequent citations."