You should check out the Merriam-Webster Dictionary of English Usage's entry
on this point. It's quite illuminating.

Here's the executive summary:

"Less" has been used with count nouns since at least the time of Alfred the
Great. In 1770, Robert Baker expressed a personal preference for "fewer" with
count nouns, and later generations of writers erected the distinction into a
hard rule.

So the failure to observe the less/fewer distinction is the persistence of old
ways of doing things. To the extent that people observe it, it is always the
result of formal education, either directly or indirectly (assimilated from
observing written examples), and not the result of any new shift in the language.

Note, btw, that other quantifiers (e.g., "all") can be used with either mass
or count nouns, so the failure to observe a distinction between "less" and
"fewer" does not mean that the mass/count distinction itself is not taking place.

Karl

Scott wrote:
> Almost all of our native language is learned orally with students' 
> learning a written dialect that is more formal than our common
> speech.  Although count/noncount is basically learned orally,
> more and more speakers seem not to have learned to distinguish
> between less and fewer thereby requiring that it be taught in school.
> My students seemed fascinated by fewer than five gallons of gas when
> you can carry just so many gallon cans and less than five gallons when 
> you are measuring it at the gas pump
> 
> 
> Scott Catledge
> Professor Emeritus
> 
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