That's exactly what the pin-pen merger is! As you know, it's not limited to pin/pen, and hearing ability (in my case, profound hearing loss) is not related to the ability to hear the difference. I don't understand the linguistics, but my very bad understanding is that there's actual brain chemistry here that means that you _can't_ hear the difference because you never learned it, never spoke it, and you pronounce them the same.

My partner is from the PNW and she pronounces "egg" as "ayg" (like "ayyyy-g") but when I say "egg" she can't hear the difference between what I'm saying and what she says. And she has perfect hearing. But she CAN hear the difference between "pin" and "pen", and she gets upset when i say them the same way. lol

But yeah, that's one of the things that makes accents accents. It's not just the sounds that come out of our mouths but the way we hear things, too. Kinda crazy. :)

When I was listening to some of the samples on the page you linked (pronunciation of “when”), it really seemed to me like the difference they were highlighting was how much the “h” was pronounced. Even knowing what I was listening for, it was very like my brain was just refusing to recognize the vowel sound distinction. So I think you must be right about it being a matter of basic brain chemistry.

In the example of the reverse pen/pin merger (HMS Pinafore) on that page, I couldn’t hear “penafore” to save my life. Fascinating stuff.

I used to think of the movie “Fargo” and think “haha comical upper midwestern accents.” And then at some point I realized that the characters in “No Country for Old Men” probably must sound similarly ridiculous to anyone whose grandparents and great grandparents didn’t all speak with a deep, rural West Texas accent - which mine did, so watching the movie it just seemed completely natural for the place and time at a deeply subconscious level.

They are the same phoneme for me in US Eastern suburbia, the only difference is in a subtle shift in the length that you drag it out. "merry" is faster than "marry" which is sometimes but not always faster than "Mary". Most UK accents seems to drag the proper name out an additional beat, and for some of them there's a slight pitch shift that sounds like "ma-ery", at its most extreme in Ireland (this is one early shibboleth by which I recognized Irish people before I really picked up on the other parts of the accent).

As someone with a German accent, to me the difference between merry and marry is the same as between German e (in this case ɛ in ipa) and ä (æ in ipa). Those two sounds are extremely close, but not quite the same. According to the Oxford dictionary that is true in British English, while it shows the same pronunciation (ɛ) for both in American English