Q: There’s a grammatical quirk in northern New England in which a negative is used affirmatively: Example: “I love it when the leaves turn in the fall.” … “Oh, so don’t I. It’s my favorite time of year.” Any ideas where that might have come from?
A: You’re right that this quirky use of “so don’t I” is peculiar to New England. A native Bostonian would understand it immediately as meaning “so do I,” while a Californian would probably hear just the opposite—“I don’t.”
The linguist William Labov has said this use of “so don’t I” represents a “reversal of polarity,” a kind of construction in which “negative comes to mean positive or positive negative.” (From his 1974 paper “Linguistic Change as a Form of Communication.”)
Labov, an expert in the fields of sociolinguistics and regional variation, says the usage is common to eastern New England. It has also been called “the Massachusetts negative positive,” and research has shown that it extends into Maine.
He and his colleagues conducted a study in which subjects were given this question: “Somebody said, I like liver and then somebody else said, So don’t I. What do you think he meant?”
A majority of those from outside eastern New England interpreted the answer in the negative: “I do not.” But all the native New Englanders interpreted it as positive: “I do too.”
As Labov notes, “So don’t I has risen to the level of an overt stereotype in eastern New England.” However, “most outsiders are puzzled by the apparent contradiction between the positive so and the negative n’t.”
The usage consists of the adverb “so,” followed by a negative auxiliary verb (“don’t,” “didn’t,” “can’t,” “couldn’t,” etc.), and a noun or pronoun subject.
It’s always spoken in response to an affirmative statement. And despite the negative “-n’t,” the speaker is being affirmative too.
Labov notes a similarity with a “tag question” that’s another form of reverse polarity: “Don’t I though!”
Another similar usage has been noted by the Yale University linguist Laurence R. Horn. In Smoky Mountain English, someone who responds to a suggestion or invitation by saying, “I don’t care to” actually means “I don’t mind if I do” or “I’m pleased to.”
As Horn writes, this usage is as likely as “so don’t I” to be “misinterpreted by outlanders.” (From his paper “Multiple Negation in English and Other Languages,” 2010.)
Jim Wood, another Yale linguist, argues that there’s a shade of difference between a New Englander’s affirmative “so don’t I” and a straightforward “so do I.” A speaker who responds with “so don’t I,” he says, is correcting an assumption.
In his paper “Affirmative Semantics with Negative Morphosyntax” (2014), Wood uses the following exchange to illustrate his point. Speaker A: “I play guitar.” Speaker B: “Yeah, but so don’t I.”
Here Speaker A seems to imply he’s the only one (that is, in the relevant context) who plays the guitar. Speaker B’s response sets him straight, and can be seen as meaning “It’s not true that I don’t play the guitar too.”
Wood, as a native of southern New Hampshire, has firsthand experience of the usage. He (along with Horn, Raffaella Zanuttini, and others) collaborated on a broad-ranging language study, the Yale Diversity Project, which researched several dozen usages in addition to “so don’t I.”
The study found that “so don’t I” had been recorded as far north as York, ME, as far south as New Haven, CT, and as far west as Erie, PA.
You can read more online about the Yale study’s “so don’t I” research, and see a map plotting its usage.