During September, Glossophilia is looking at word pairs that often get muddled up with each other, or that essentially mean the same thing. Today’s is underestimate vs. overestimate.

The American conductor Marin Alsop, addressing the audience after leading the Last Night of the Proms earlier this month, said: “The power of music can’t be underestimated. It’s what makes us human beings.” And no, this isn’t a post about underestimating the British capacity for patriotism, which reveals itself most notably but once a year at this stirring Britannic Union Jack-fest. It’s about the words she used.

Did Alsop really mean to say “can’t be overestimated” — as Frank Fahy suggested in his letter to The Guardian on Sep 8? “I am used to hearing sports commentators saying underestimate when they mean overestimate, but to hear Marin Alsop at the Last Night of the Proms opine that “You cannot underestimate the power of music” was disappointing.” Or was her choice of words perfectly in keeping with common usage and idiom? Certainly there was no doubt about the meaning of the message she intended to convey.

Many will argue that underestimate frequently replaces overestimate erroneously and illogically. According to Collins English Dictionary, “underestimate is sometimes wrongly used where overestimate is meant: the importance of his work cannot be overestimated (not cannot be underestimated).” Paul Brians in his Common Errors in English Usage agrees, repeating Fahy’s observation when he notes that “enthusiastic sportscasters often say of a surprisingly talented team that ‘they cannot be underestimated’ when what they mean is ‘they should not be underestimated’.” (What’s with the sports commentators and their over-enthusiasm for underestimating?)

It’s interesting to note that what all these apparently erroneous statements have in common is that they pair cannot with underestimate. Could that be where the problem lies: in this quasi double-negative? (Underestimate isn’t really a negative as such, but the phrase “cannot underestimate” requires a certain process of logical thought that involves thinking about the opposite of both words.) A simple statement that doesn’t involve the negative cannot — such as “I completely underestimated her ability” (ie. she was far more able than I had expected her to be) — presents no ambiguity or confusion, and the opposite statement (“I completely overestimated her ability”, ie. oh boy: I thought she was going to be more capable) is equally unambiguous. But when cannot enters the mix, confusion sets in. A very rudimentary Google search on “can’t be overestimated” returns hundreds of thousands of results; a similar search using underestimated instead returns well over a million results — with the same meaning intended. This is clearly today’s preferred idiom, illogical as it might seem. Why?

The Language Log blog addressed this question a few years ago in a post called “Weird Logic and Bayesian Semantics”, examining several common English expressions — “could care less”, “still unpacked”, “(not) fail to miss”, and “cannot be underestimated” — that are “almost always used as if their meanings had a negation added or subtracted. … It’s especially hard when some of the negatives are implicit in word meanings (like fail or miss), or when modals (like can or possible) and scalar expressions (like less or still or underestimate) are also involved.” The post’s author, Mark Liberman, offers what he describes as “Bayesian Semantics” logic to explain the phenomenon. In a nutshell, and based on the notion that “Nature (or at least the Speech Community) abhors a semantic vacuum”, Liberman suggests that because the literal meaning of “it is not possible to underestimate X” is something that people are extremely unlikely to intend to convey, the phrase “you can’t underestimate” simply assumes the more probable meaning (ie. “you can’t overestimate”) to fill the semantic void.

But there’s another argument, also discussed by Liberman, that’s easier to understand and is perhaps the more likely explanation for this deviant idiom. It acknowledges an accepted colloquial meaning of can’t (or cannot), which can sometimes be defined loosely as “may not”, “must not”, or “should not”. When a mother reprimands her son, for example, and tells him that he “can’t” do something, what she really means is that he “should not” or “is not allowed” to do it, rather than that he is incapable of committing the forbidden act. So by the same token, the statement that you “can’t underestimate” something can really mean that you “must not underestimate” it.

Harold Somers (who, incidentally, is a Professor [emeritus] of language engineering at the University of Manchester) offers perhaps the most persuasive defense of Alsop’s statement and of our preferred-though-technically-incorrect idiom. A couple of days after Fahy’s letter of disappointment appeared in The Guardian, the paper published Somers’s response: “Your correspondent (Letters, 9 September) is wrong to criticise Marin Alsop for saying: “You cannot underestimate the power of music.” There is a use of the word “can” in the negative that has the same meaning as “should not”, as in “You cannot eat too much chocolate.” It gives rise to a nice ambiguity, as in “You cannot make too many sandwiches (because they will go to waste)”/”You cannot make too many sandwiches (because they will all get eaten however many there are)”. All I can say is that you cannot underestimate the subtlety of the English language, except at your own risk.”

First posted Sep 2013.