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.”
Two earlier blog posts [1, 2] mentioned this in passing; this post explores the use of et al. in relation to the number of authors in some detail.
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: “All authors must be listed; never use ‘et al.’ or the phrase ‘or others’ ”. 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 second category, the APA suggests listing the first three names followed by et al. when the paper is cited for the first time. For the second and subsequent citations, it 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.
 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.
["Publish and prosper" is a series of posts about tips for researchers whose first language is not English but who submit papers to journals published in English. The series touches upon not only writing (spelling, grammar, punctuation, usage, and style) but everything else relevant to publishing research papers that journal editors wish their authors knew.]