You've indicated that you use the apa
bibliography style. I suggest you proceed as follows to achieve your formatting objective:
Find the file apa.bst
in your TeX distribution. Make a copy of this file and call the copy, say, apa-erel.bst
.
Open the file apa-erel.bst
in a text editor. (The editor you use for the main tex file will do fine.
In the file apa-erel.bst
, locate the function format.lab.names
. (In my copy of this file, the function starts on line 866.)
Delete (or comment out) all 17 or so lines of this function and replace them with the following code:
FUNCTION {bbl.and}
{ "and"}
FUNCTION {space.word}
{ " " swap$ * " " * }
FUNCTION {format.lab.names}
{'s :=
"" 't :=
#1 'nameptr :=
s num.names$ 'numnames :=
numnames 'namesleft :=
{ namesleft #0 > }
{ s nameptr
"{vv~}{ll}" format.name$
't :=
nameptr #1 >
{
nameptr #2 =
numnames #3 > and
{ "others" 't :=
#1 'namesleft := }
'skip$
if$
namesleft #1 >
{ ", " * t * }
{
s nameptr "{ll}" format.name$ duplicate$ "others" =
{ 't := }
{ pop$ }
if$
t "others" =
{ " et~al." * }
{
numnames #2 >
{ "," * }
'skip$
if$
bbl.and
space.word * t *
}
if$
}
if$
}
't
if$
nameptr #1 + 'nameptr :=
namesleft #1 - 'namesleft :=
}
while$
}
Save the file apa-erel.bst
, either in a directory that's searched by BibTeX or in the directory where your main tex file is located. If you choose the former method, be sure to update the filename database of your TeX distribution suitably.
In your main tex file, change the instruction \bibliographystyle{apa}
to \bibliographystyle{apa-erel}
and perform a full recompile cycle -- latex, bibtex, latex, latex -- to fully propagate all changes.
Happy BibTeXing!
A full MWE:

\RequirePackage{filecontents}
\begin{filecontents}{mybib.bib}
@misc{ab, author = "A and B", title = "X", year = 3001, }
@misc{abc, author = "A and B and C", title = "Y", year = 3002, }
@misc{abcd, author = "A and B and C and D", title = "Z", year = 3003, }
\end{filecontents}
\documentclass{article}
\usepackage[authoryear,round]{natbib}
\bibliographystyle{apa-erel}
\begin{document}
\cite{ab}
\cite{abc}
\cite{abcd}
\bibliography{mybib}
\end{document}
biblatex
? It's just a matter of changing the values of the keysmaxcitenames
andmincitenames
.