The troublesome cases are when we have one term that can refer to either sex, and another that refers only to women. Take the word “author.” It can refer to men or women in a sentence like “Our agency represents many authors.” But if you use “author” to refer to writers of either sex, and the exclusively feminine “authoress” to refer to female authors, you now have a way of referring specifically to female authors, but no way of referring specifically to male authors.
Read the rest of the post at When to Use Female Nouns