It is common to cite studies by referring to the author’s last name. For most names, this is easy. For instance, if Smith published a study in 2013, we can say “Smith’s (2013) study.”
But what happens when the person’s last name ends with the letter s, like the name Spears? Should it be “Spears’ (2013) study” or “Spears’s (2013) study”?
You might be surprised, but the correct version is “Spears’s (2013) study.”
Take, for instance, the article by Laurin, Kay, and Fitzsimons in the January 2012 issue of the Journal of Personality and Social Psychology. In the text, you might write,
If the additional s seems strange, you are probably thinking of how we normally handle plural nouns. In those cases, we do not add an s. That is, we would write “The participant’s sociodemographic characteristics” to refer to the characteristics of just one participant, but we would write “The participants’ sociodemographic characteristics” to refer to the characteristics of multiple participants.
The difference between these two guidelines is that Fitzsimons’s last name is not plural and it’s not a common noun–it’s a singular, proper noun. Even if you can’t remember the grammatical terms, just remember that for names that end in s, you still have to add an s after the apostrophe.
For more examples, see section 7.16 in the Chicago Manual of Style, 16th edition.
Just as a side note, you don’t have to use the possessive form to refer to your studies. You can also say “A study by Laurin, Kay, and Fitzsimons (2012)” or refer to it in some other way. But if you do want to use the possessive, you can now avoid a common error, even among seasoned academic writers.