Grammar Peas...please answer this!

Per APA Style, the answer is that the possessive of a singular name is formed by adding an apostrophe and an s, even when the name ends in s (see p. 96 in the sixth edition of the Publication Manual). Therefore, in the example above, the correct usage would be “Adams’s (2013) work.” Although this presentation may look awkward to some writers, the rule for forming the possessive does not change just because the name ends in s.

However, it is important to note the following exception to this rule: You should use an apostrophe only with the singular form of names ending in unpronounced s (see p. 97 in the Publication Manual). Therefore, if you were writing a paper about the philosopher Descartes, to form the possessive with his name, you would need to just add an apostrophe (e.g., Descartes’ theory).

To help illustrate these guidelines, let’s look at a few more examples of properly formatted possessives:

Sigmund Freud’s method
Jesus’s disciples

Charles Dickens’s novels

Socrates’s life

François Rabelais’ writings (note that Rabelais ends with an unpronounced s)