Citations with same first author and date with apa style: no letter generated

Question

I use the package biblatex with biber to generate my bibliography. I have some references with the same first author and the same year but no letter is inserted to make the distinction possible. I saw the same problem here with biblatex-chicago.

Here is my code:

\documentclass[american]{book}

\usepackage[utf8]{inputenc}
\usepackage[autostyle]{csquotes}
\usepackage[american]{babel}

\usepackage[backend=biber,
uniquename=false,
uniquelist=false,
style=apa,
doi=false,
isbn=false,
url=false,
maxcitenames=3,
uniquelist=false,
natbib]{biblatex}

    %%Bibliography format
\addbibresource{test.bib}
\DeclareLanguageMapping{american}{american-apa}
\AtEveryBibitem{\clearfield{labelmonth}}
\AtEveryBibitem{\clearfield{note}}
\DefineBibliographyStrings{american}{bibliography = {Bibliographie}, references = {Références},}

\begin{document}

An example of citation: the first time, all authors \parencite{Saffran1996}, \parencite{Saffran1996a}.

After the first time, citations are simplified \parencite{Saffran1996}, \parencite{Saffran1996a}.

But no letter are inserted to distinguish the citations...

\printbibliography
\end{document}

with this test.bib file :

% This file was created with JabRef 2.10.
% Encoding: ISO8859_1


@Article{Saffran1996a,
  Title                    = {Statistical Learning by 8-Month-Old Infants},
  Author                   = {Saffran, Jenny R. and Aslin, Richard N. and Newport, Elissa L.},
  Year                     = {1996},
  Doi                      = {10.1126/science.274.5294.1926},
  ISSN                     = {0036-8075, 1095-9203},
  Language                 = {en},
  Month                    = dec,
  Note                     = {{PMID:} 8943209},
  Number                   = {5294},
  Pages                    = {1926--1928},
  Url                      = {http://www.sciencemag.org/content/274/5294/1926},
  Urldate                  = {2014-01-19},
  Volume                   = {274},

  Abstract                 = {Learners rely on a combination of experience-independent and experience-dependent mechanisms to extract information from the environment. Language acquisition involves both types of mechanisms, but most theorists emphasize the relative importance of experience-independent mechanisms. The present study shows that a fundamental task of language acquisition, segmentation of words from fluent speech, can be accomplished by 8-month-old infants based solely on the statistical relationships between neighboring speech sounds. Moreover, this word segmentation was based on statistical learning from only 2 minutes of exposure, suggesting that infants have access to a powerful mechanism for the computation of statistical properties of the language input.},
  File                     = {Saffran, Aslin & Newport (1996) Statistical Learning by 8-Month-Old Infants.pdf:/Users/Croche_8/Library/Application Support/Zotero/Profiles/bmql718o.default/zotero/storage/87H7TDZF/Saffran, Aslin & Newport (1996) Statistical Learning by 8-Month-Old Infants.pdf:application/pdf;Snapshot:/Users/Croche_8/Library/Application Support/Zotero/Profiles/bmql718o.default/zotero/storage/7X7QIAE7/1926.html:text/html},
  Journal                  = {Science},
  Owner                    = {Croche_8},
  Timestamp                = {2014.03.27}
}


@Article{Saffran1996,
  Title                    = {Word Segmentation: The Role of Distributional Cues},
  Author                   = {Saffran, Jenny R. and Newport, Elissa L. and Aslin, Richard N.},
  Year                     = {1996},
  Doi                      = {10.1006/jmla.1996.0032},
  ISSN                     = {0749-{596X}},
  Month                    = aug,
  Number                   = {4},
  Pages                    = {606--621},
  Url                      = {http://www.sciencedirect.com/science/article/pii/S0749596X96900327},
  Urldate                  = {2014-03-18},
  Volume                   = {35},

  Abstract                 = {One of the infant's first tasks in language acquisition is to discover the words embedded in a mostly continuous speech stream. This learning problem might be solved by using distributional cues to word boundaries—for example, by computing the transitional probabilities between sounds in the language input and using the relative strengths of these probabilities to hypothesize word boundaries. The learner might be further aided by language-specific prosodic cues correlated with word boundaries. As a first step in testing these hypotheses, we briefly exposed adults to an artificial language in which the only cues available for word segmentation were the transitional probabilities between syllables. Subjects were able to learn the words of this language. Furthermore, the addition of certain prosodic cues served to enhance performance. These results suggest that distributional cues may play an important role in the initial word segmentation of language learners.},
  File                     = {ScienceDirect Full Text PDF:/Users/Croche_8/Library/Application Support/Zotero/Profiles/bmql718o.default/zotero/storage/3WG62WGP/Saffran et al. - 1996 - Word Segmentation The Role of Distributional Cues.pdf:application/pdf;ScienceDirect Snapshot:/Users/Croche_8/Library/Application Support/Zotero/Profiles/bmql718o.default/zotero/storage/FRRTV6HA/S0749596X96900327.html:text/html},
  Journal                  = {Journal of Memory and Language},
  Owner                    = {Croche_8},
  Shorttitle               = {Word Segmentation},
  Timestamp                = {2014.03.27}
}

which gives the following result:

Question:

How could I generate letters to distinguish the citations with the same first author/date ?

Answer 1

Instead of biblatex use directly natbib (e.g. with apalike style), it does this automatically. And it does not matter if the 2nd and 3rd authors are in different order. It has also a lot of formating possibilities, see the manual

The test.bib looks like this (I cut it a bit just for saving space):

@Article{Saffran1996a,
  Title                    = {Statistical Learning by 8-Month-Old Infants},
  Author                   = {Saffran, Jenny R. and Aslin, Richard N. and Newport, Elissa L.},
  Year                     = {1996},
  Doi                      = {10.1126/science.274.5294.1926},
  ISSN                     = {0036-8075, 1095-9203},
  Language                 = {en},
  Month                    = dec,
  Note                     = {{PMID:} 8943209},
  Number                   = {5294},
  Pages                    = {1926--1928},
  Url                      = {http://www.sciencemag.org/content/274/5294/1926},
  Urldate                  = {2014-01-19},
  Volume                   = {274}
}

@Article{Saffran1996,
  Title                    = {Word Segmentation: The Role of Distributional Cues},
  Author                   = {Saffran, Jenny R. and Newport, Elissa L. and Aslin, Richard N.},
  Year                     = {1996},
  Doi                      = {10.1006/jmla.1996.0032},
  ISSN                     = {0749-{596X}},
  Month                    = aug,
  Number                   = {4},
  Pages                    = {606--621},
  Url                      = {http://www.sciencedirect.com/science/article/pii/S0749596X96900327},
  Urldate                  = {2014-03-18},
  Volume                   = {35},
  Journal                  = {Journal of Memory and Language}
}

and in the .tex file put:

\documentclass[american]{book}

\usepackage[utf8]{inputenc}
\usepackage[american]{babel}

\usepackage{natbib}

\begin{document}

An example of citation: in the text \citet{Saffran1996a}, \citet{Saffran1996}.

In brackets \citep{Saffran1996}, \citep{Saffran1996a}.

\bibliographystyle{apalike}
\bibliography{test}

\end{document}

The difference is only in defining the bibliography and the \citep, \citet or \cite commands.

The output is: