As late as 1777, when the Royal Standard Dictionary was published, the predominant pronunciations of “either” and “neither” in England were “ee-ther” and “nee-ther.” But that gradually changed. In its 1907 edition, The Oxford Dictionary remarked that “eye-ther” was more prevalent in the “educated speech” of Londoners. H.W. Fowler predicted in 1926 that this pronunciation would “probably prevail,” and by 1965, when Sir Ernest Gowers revised Fowler’s A Dictionary of Modern English Usage, it had “almost wholly displaced” the long e pronunciation.
For a long time, the United States stuck with “ee-ther,” for the most part. In 1873, the philologist W.D. Whitney harrumphed that the ”eye-ther” pronunciation had “spread …by a kind of reasonless and senseless infection, which can only be condemned and ought to be stoutly opposed and put down.” A 1928 satirical sketch called “The Lady Buyer” noted of that personage,
always, standing her in good stead, and ready at the tip of her tongue is her crystal-clear, British pronunciation of “either.” She says the staunch word with such hauteur as to make one forget other mistakes and even feel apologetic for having noticed them. Nothing on earth could make her whisper “ether” in the darkest corner of a stock-room. She knows it would ruin her socially.
Memorably, “eye-ther” was one of the British pronunciation choices (along with “to-mah-to”) in Ira Gershwin’s 1937 lyric to “Let’s Call the Whole Thing Off.” In 1961, Hans Kurath and Raven McDavid called “eye-ther” “a sporadic feature of the cultivated speech of Metropolitan New York and Philadelphia…. it is in all probability a recent adoption from British English.”
My ears tell me is that “eye-ther” and “nIe-ther” are currently on the rise in America, especially among young people. My own millennial-generation daughter, despite having two parents who say “ee-ther,” says “eye-ther.” On the pronunciation site Youglish, six of the first twenty American utterances of the word are “eye-ther” (all youngish people), and fourteen are “ee-ther,” including Barack Obama and Hillary Clinton.
I would be interest in the observations of readers, both American and British–or any other English-speaking country, for that matter.