Oxford comma only for three or more authors, but not for exactly two authors

Question

I want to insert the Oxford Comma only if an entry has 3 or more authors, but not if the entry has exactly two authors.

e.g.

A and B (1999) [NO COMMA]

A, B, and C (1999) [COMMA before C]

I tried

author = "A and Firstname { B,} and C"

in the bib entry itself. This works for citation call-outs, but then two commas appear in the formatted bibliographic entry.

I use the aea bibliography style, which is available online here. As you can probably guess by now, this bibliography style is set up to always insert a comma between penultimate_author and and final_author -- even if an entry has exactly two authors. Is there a way to adjust this bibliography style such that the Oxford comma gets inserted only if an entry has three or more authors?

For the sake of completeness, here's the function format.names from the aea bibliography style:

FUNCTION {format.names}
{ 's :=
  #1 'nameptr :=
  s num.names$ 'numnames :=
  numnames 'namesleft :=
  { namesleft #0 > }
    { nameptr #1 = 
        {   s nameptr "{vv~}{ll}{, jj}{, ff}" format.name$ 't := }
        {   s nameptr "{ff }{vv~}{ll}{, jj}" format.name$ 't := }
        if$
      nameptr #1 >
        { namesleft #1 >
          { ", " * t * }
          { t "others" =
            { ", et~al." * }
            { " and " * t * }
            if$
          }
          if$
        }
        't
        if$
      nameptr #1 + 'nameptr :=
      namesleft #1 - 'namesleft :=
    }
    while$
}

Answer 1

I suggest you proceed as follows:

• Make a copy of aea.bst and call the copy, say, aea-oxford.bst.

• Open the file aea-oxford.bst in a text editor. The program you use to edit your tex files will do fine.

• In the file, locate the function format.names. Basically, that's the function whose code you displayed in your query.

• In this function, replace the three lines

    { t "others" =
      { ", et~al." * }
      { ", and " * t * }
  

  with the following seven lines:

        { numnames #2 >
            { "," * }
            'skip$
          if$
          t "others" =
            { " et~al." * }
            { " and " * t * }
  

  The first part of the new code consists of a test to check if the total number of surnames is greater than 2. If yes, insert a comma; if no, do nothing. In the second part (which only ever gets executed if there are two or more surnames, one of which may be the string "others"), a plain "and" is inserted if the second surname is "real", i.e., doesn't consist of the string "others".

• Store the file aea-oxford.bst in the directory where your main tex file is located.

• In the main tex file, change the argument of \bibliographystyle from aea to aea-oxford. Then, perform a full recompile cycle -- LaTeX, BibTeX, LaTeX, and LaTeX once more -- to fully propagate all changes.

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A full MWE (minimum working example) and screenshot; I've highlighted in yellow the two changes wrought in aea-oxford.bst:

\RequirePackage{filecontents}
\begin{filecontents}{mybib.bib}
@misc{a, author = "A", title  = "Thoughts", year   = 3001, } 
@misc{ab, author = "A and B", title  = "Thoughts", year   = 3002, }
@misc{abc, author = "A and B and C", title  = "Thoughts", year   = 3003, }
@misc{abcd, author = "A and B and C and D", title  = "Thoughts", year   = 3004, }
@misc{abcde, author = "A and others", title  = "Thoughts", year   = 3005, }
\end{filecontents}

\documentclass{article}
\usepackage[round,authoryear]{natbib}
\bibliographystyle{aea-oxford}

\begin{document}
\noindent
\cite{a}, \cite{ab}, \cite{abc}, \cite{abcd}, \cite{abcde}
\bibliography{mybib}
\end{document}

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Just for comparison, here's the output of the same MWE if the unmodified aea bibliography style were used -- note the two additional commas: