final devoicing in American English?

[Alice Faber]

Interesting though the 10 (+/- 3) commandments are, I've got a question
pertaining to the more conventional core mission of this list.

I'm teaching an undergraduate course that involves some phonetic
transcription (Intro to Psycholinguistics, to students who haven't had an
Intro to Linguistics) for the first time in a very long time. Something
came up in class today that I haven't previously encountered. I was doing
the standard presentation of regular English plurals, so that the students
could discover the phonological conditioning of /-s/, /-z/, and /-Iz/
variants. To my surprise, several of the students insisted that /z/ in
_dogs_ is at least partially voiceless. They weren't getting tricked by the
orthography. (Many of them were quite positive that _tree_ begins with an
affricate, for instance, and they were making fairly subtle vowel quality
distinctions.) Two of them, one from northern Georgia and one whose first
language is Russian (though her English sounds virtually accent-free), had
more conventional final devoicing, including regressive assimilation, so
that _dogs_ ends /ks/. These are all Wesleyan University undergrads of
normal college age.

Is this something that's been going on for a while, with me blissfully
oblivious, or is it (relatively) new?

Alice Faber