Imagine that you're back in elementary school, having a discussion on the playground with one of your classmates. It goes something like this:
You: My dad can beat up your dad!
What is the proper response here? For me, it is and has always been, "Uh-huh!" But in the past few years, I've been hearing "Yuh-huh!" At first it was just in a few TV shows (I think I've heard it in "Friends"), and then I saw it written in a comic strip or two. When I really started to take notice was when my son Doug got to be old enough to have these kinds of conversations, and always said "Yuh-huh," never "Uh-huh." That was interesting, because he certainly didn't acquire yuh-huh from me (any more than I acquired uh-huh from my parents). He must have gotten it from his peers, which meant that they were all saying yuh-huh, too. To make a hasty generalization out of it, there seems to be a generational shift from uh-huh to yuh-huh.
I asked some people about this a few years ago, and got some anecdotal support of the hypothesis. For example, here's what Glen said:
I've noticed the gradual emergence of "yuh-huh" as the response of choice. It's often been used in Buffy the Vampire Slayer by Dawn, Buffy's 15-year-old younger sister. The younger someone is, the more likely they are to say "yuh-huh" instead of "uh-huh." But I'm old enough that I still prefer "uh-huh."
And another guy told me that he used to say uh-huh, but picked up yuh-huh from his kids.
When I did an Internet search, I found:
several attestations of nuh-uh and uh-huh close to each other
many more attestations of nuh-uh and yuh-huh close to each other, the oldest of which seems to be a 2000 episode of The Simpsons (though there are a couple of scripts from Friends episodes that might be older).
no cases in which a person uses both uh-huh and yuh-huh in response to nuh-uh
several cases in which a person uses both uh-huh and yuh-huh, but in these cases, uh-huh is always the conversation-continuing particle, not an emphatic affirmation
an entry for yuh-huh (but not uh-huh) in a listing of (I think) Pittsburgh English vocabulary
The last item was interesting, since it meant that maybe yuh-huh was more of a regional thing, which was now spreading.
But the most intriguing hypothesis on the origin of yuh-huh came from a comment from my parents: Although I've been using nuh-uh since I was a little kid, even it seems to have come on the scene within the last two generations or so. Before nuh-uh, there was uh-uh (also written unh-uh or unh-unh) If nuh-uh arose as a blending of uh-uh and no, then maybe yuh-huh is just a delayed analogical blending of uh-huh and yeah. Schematically:
uh-uh — merge with no --> nuh-uh
uh-huh — merge with yeah --> fill in the blank
Analogy is known to be a powerful force in language change (see for example my previous posts on backformation, here and here), but as for whether the scenario above is actually what happened, I don't know.