This problem is due to biblatex-apa's (apa.bbx's) apaauthor name format; you might want to notify the author of this small bug.
The string and others in the author list sets ifmorenames to true. apaauthor checks for this case whenever a name is printed, not just at the very end of the list; consequently it prints the andothers string ("et al.") after each name.
The fix is to add the following lines to your preamble.
\DeclareNameFormat{apaauthor}{%
\ifthenelse{\value{listcount}=\maxprtauth\AND\value{listcount}<\value{listtotal}}
{\addcomma\addspace\ldots\addspace}
{\ifthenelse{\value{listcount}>\maxprtauth\AND\value{listcount}<\value{listtotal}}
{}
{\ifthenelse{\iffieldequalstr{doubtfulauthor}{true}}
{\mkbibbrackets{\usebibmacro{name:apa:last-first}{#1}{#3}{#4}{#5}{#7}?}}
{\usebibmacro{name:apa:last-first}{#1}{#3}{#4}{#5}{#7}}}}%
\ifthenelse{\value{listcount}=\value{listtotal}}% this test is new
{\ifmorenames{\andothersdelim\bibstring{andothers}}{}}{}}
The MWE
\RequirePackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@Article{boker2011,
Author = {Boker, S. and Neale, M. and Maes, H. and Wilde, M. and
Spiegel, M. and Brick, T. and Spies, J. and Estabrook,
R. and Kenny, S. and Bates, T. and others},
Title = {Open{M}x: {A}n open source extended structural
equation modeling framework},
Journal = {Psychometrika},
Volume = {76},
Number = {2},
Pages = {306--317},
year = 2011
}
\documentclass{apa6}
\usepackage[american]{babel}
\usepackage{csquotes}
\usepackage[style=apa,maxnames=999,sortcites=true,sorting=nyt,apabackref=true,backend=biber]{biblatex}
\DeclareLanguageMapping{american}{american-apa}
\addbibresource{\jobname.bib}
\addbibresource{biblatex-examples.bib}
\DeclareNameFormat{apaauthor}{%
\ifthenelse{\value{listcount}=\maxprtauth\AND\value{listcount}<\value{listtotal}}
{\addcomma\addspace\ldots\addspace}
{\ifthenelse{\value{listcount}>\maxprtauth\AND\value{listcount}<\value{listtotal}}
{}
{\ifthenelse{\iffieldequalstr{doubtfulauthor}{true}}
{\mkbibbrackets{\usebibmacro{name:apa:last-first}{#1}{#3}{#4}{#5}{#7}?}}
{\usebibmacro{name:apa:last-first}{#1}{#3}{#4}{#5}{#7}}}}%
\ifthenelse{\value{listcount}=\value{listtotal}}
{\ifmorenames{\andothersdelim\bibstring{andothers}}{}}{}}
\shorttitle{IFA}
\begin{document}
OpenMx \parencite{boker2011,aksin,wilde,murray}
\printbibliography
\end{document}
then yields
Update 2013-10: This has been corrected in biblatex-apa version 6.4 according to a comment below the question from user PLK.
First thing to do is to add a field for the journal abbreviation to the article entrytype, journalabbr in the MWE. Therefore you need to declare a datamodel (an extra file; in the MWE I've used filecontents to simulate that) and you have to tell biblatex/biber to use it in the package options.
Then you have to modify the cite command, so it fits your needs. \citep from the authoryear-style (your citestyle) uses the \cite command. There you just have to add a switch, which checks if the field journalabbr is empty or not, and prints it out or not.
Last but not least you have to add the abbreviation fields to the entries.
MWE:
\begin{filecontents}{min.bib}
@article{boisson2003unexpected,
title={Unexpected protein families including cell defense components feature in the N-myristoylome of a higher eukaryote},
author={Boisson, B. and Giglione, Carmela and Meinnel, Thierry},
journal={Journal of Biological Chemistry},
journalabbr={JBC},
year={2003},
publisher={ASBMB}
}
\end{filecontents}
\begin{filecontents}{authorjabbryear.dbx}
\ProvidesFile{authorjabbryear.dbx}
\DeclareDatamodelFields[type=field,datatype=literal]{journalabbr}
\DeclareDatamodelEntryfields[article]{journalabbr}
\end{filecontents}
\documentclass[fontsize=11pt, paper=a4, ngerman, DIV=calc]{scrartcl}
\usepackage[scaled]{helvet}
\renewcommand*\familydefault{\sfdefault}
\usepackage{fixltx2e}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{babel}
\usepackage[german=quotes]{csquotes}
\usepackage[style=authoryear-comp,sortcites=true,sorting=nyt,isbn=false,natbib=true, citestyle=authoryear,bibstyle=authoryear,backend=biber,maxnames=1,maxcitenames=1,
,datamodel=authorjabbryear%added!
] {biblatex}
\DefineBibliographyStrings{ngerman}{ andothers = {{et\,al\adddot}} }
\renewbibmacro*{cite}{%from authoryear.cbx
\iffieldundef{shorthand}
{\ifthenelse{\ifnameundef{labelname}\OR\iffieldundef{labelyear}}
{\usebibmacro{cite:label}%
\setunit{\addspace}}
{\printnames{labelname}%
\setunit{\nameyeardelim}}%
\iffieldundef{journalabbr}{}{%
\printfield{journalabbr}%
\setunit{\nameyeardelim}%
}%
\usebibmacro{cite:labelyear+extrayear}}
{\usebibmacro{cite:shorthand}}}
\addbibresource{min.bib}
\begin{document}
\citep{boisson2003unexpected}
How it should look like:
(Boisson et al., JBC, 2003)
\printbibliography
\end{document}
Best Answer
A possible solution is to introduce a toggle to determine whether we are in a text cite or not. Then the modification of
\mkbibnamelastis parametrised to the toggle. This solution uses thexpatchpackage to modify thetextcitebibmacro, to set the toggle to true just before executing thetextcitemacro and to false right after thetextcitemacro has been executed.