This year has been an unusually busy one in terms of global activism. First, it was the Arab Spring that saw Middle Eastern countries erupting in anti-government protests. Then came the American Autumn and the Occupy Wall Street protests. And the entire time, we being eagle-eyed grammar nerds here at The Atlantic Wire wondered who was taking to the streets--was it protesters or protestors? Reading the newspaper didn't help one bit. Everyone from The Guardian to The Washington Post showed inconsistent use of both the -er and the -or version. The New York Times even used both in the same article while covering Yemen. Are both spellings equally as acceptable?
We'll cut to the chase. According the current editions of both the Merriam-Webster and the Oxford English Dictionaries, both spellings are correct. However, both volumes list protester before protestor, indicating that the -er spelling is preferred. This is why the Associated Press decided to list protester as the correct spelling in the AP Stylebook. We cross-checked AP's guideline with British equivalents, and both The Guardian and The Times of London list protester as the preferred spelling over protestor.
"We use -er in our style and that's been our style for some time," says AP Stylebook co-editor David Minthorn, who's been at the organization since 1969. "Our general guidance when we go to the dictionary is to take the first spelling in Webster's. Now that may have been one of our criteria for using the -er spelling, but in any case that's our ruling."
All that said, there's an interesting little wrinkle in the grammar rules that govern words with both an -er and -or ending. According to John Simpson, chief editor of the Oxford English Dictionary, the variation tends to indicate a level of expertise.
"The suffix -er is the regular one used in English for agent nouns (nouns for people who do things, like protest)," Simpson writes in an email to The Atlantic Wire. "If we leave aside (a) words formed on verbs in -ate, where -or is normal, and (b) words in which the stem isn't a verb at all, like doctor, then I think that the general feeling is that -or implies a rather specialized, technical, or professional role (as with advisor in contrast to adviser). But I should stress that this is only a tendency--one finds instances of both spellings."
While our very own style guide calls for the "protester" spelling, like the rest of the journalists out there we've slipped up in the past, as well. With some etymological oomph to back it up, however, the Occupy Wall Street veterans can consider it a compliment to their professionalism if we make the same typo in the future.