Most researchers are concerned with how to write references mainly because ethical publication demands that authors provide an inclusive list of their sources. Failure to meet these publication requirements could result in the author being penalized for plagiarism, the research paper being denied publication, and/or the author being barred from submitting to a particular publisher for a few years. Although, readers might merely glance through the references section, it serves as their guide for further reading and research on similar topics, while simultaneously reflecting the extent of the author’s literature review. Researchers writing their first research paper would have come across the term “et al.” in many research papers they’ve read, but they might not understand what it means or the nuances of et al usage. Here I provide some simple guidelines that will help you remember how to use et al.
It is rare for a single researcher to publish a paper all by herself or himself; most research papers have several or many authors, and the average number of authors for a paper keeps rising as science becomes increasingly collaborative. In citing such papers using the name-and-date system, also known as the Harvard system, a long string of names proves awkward—which is why most journals recommend the use of et al., which is Latin for et alii or et aliae and means “and others.”
Despite gaining an understanding of the meaning of et al., authors are often still confused about how to write et al. Essentially, the second word of the term et al takes on a ‘period’ (et al.) irrespective of whether it falls in the middle of a sentence. How many authors should a paper have before its citation is shortened by using et al.? Journals differ widely in this respect, and the American Journal of Tropical Medicine and Hygiene avoids the usage altogether, no matter how long a citation may run to. How to Write and Publish a Scientific Paper  summarizes current practice as using et al. for more than three authors, a practice also favored by the Chicago Manual of Style , whereas Cambridge University Press, for its STM (science, technology, medicine) books, says: “Citations to works with three authors can either: (1) give all three names the first time, and thereafter use et al.; (2) give all three names for every citation; or (3) use et al. throughout” but “Citations to works with four or more authors should use et al. throughout” .
The American Psychological Association (APA)  has an even more elaborate system that uses three categories of papers: those with one or two authors, those with three to five authors, and those with six or more authors. For citations that fall into the first and second categories, the APA suggests listing all the names when the paper is cited for the first time. For subsequent citations, the style for the first category remains the same; for the second category, APA suggests shortening the citation to the name of the first author followed by et al. Papers with six or more authors are always cited giving the name of the first author followed by et al.
Therefore, it is best to style in-text citations after studying the instructions to authors of your target journal as well as its recent issue.
- Et al. (n.d.). Retrieved August 01, 2016, from http://grammarist.com/usage/et-al/ (With regard to the sentence in bold)
 Day R A and Gastel B. 2006. How to Write and Publish a Research Paper, 6th edn. Westport, Connecticut: Greenwood Press. 320 pp.
 University of Chicago Press. 2010. The Chicago Manual of Style, 16th edn. 1026 pp.
 American Psychological Association. 2009. Publication Manual of the American Psychological Association, 6th edn. Washington, DC: APA Press. 272 pp.
Wouldn't it be useful to know about Latin phrases in scientific writing: italics or not?
Correction: An earlier version of the post stated that for citations that fall into the second category, the APA suggests listing the first three names followed by et al. at first mention. This has been corrected to clarify that the APA suggests listing all the names at first mention.