Which is correct grammar? A growing share of adults lack insurance/A growing share of adults lacks insurance.?

I am trying to solve an office grammar dispute using the verb lack or lacks. One co-worker says the correct usage is "lacks" because the word "share" is used as a singular noun and that is the sentence's main subject. I say that the correct useage is "lack" because the word "adults" is plural and I think "adults" is the main subject and "growing share" is just describing the "adults" in the snetence. Please help me with any grammar rules that you may know to solve our dispute.

lacks is correct. The word "of" is a preposition, making the "of adults" a prepositional phrase. Any prepositional phrase in a sentence is there just to qualifty (define, describe - like an adjective) the noun (share in this case). That means "share" is the noun (subject) of the sentence so the verb needs to be "lacks" to mach the sigular noun. If you take out the prepositional phrase, the sentence would be "A growing share lacks insurance" - the adults just helps tell the reader what share you are talking about.

Your co-worker is correct.

Instinctively, "lack" sounds better because adults comes right before it; however, "adults" is not the subject of the sentence. It's a mere detail, a part of the prepositional phrase "of adults". It is not essential to the sentence. Grammatically, "of adults" is describing "share".

Take out "of adults" and the sentence still makes sense. A growing share lacks insurance.

Take out "share", the subject, and you don't even have a sentence.

Technically, the subject would be "share" with "of adults" being a prepositional phrase modifying "share." "Share" is singular, but it's a collective noun and the rules can sometimes get hazy on these--moreso in the U.K. than in America.

Here's a wikipedia article that might apply:
http://en.wikipedia.org/wiki/Synesis

So, the answer is that there's no clear answer. I would recommend avoiding the whole mess by rephrasing the sentence. Maybe:
"The number of adults without insurance is growing."
or
"The percentage of adults without insurance is growing."
:)

the subject in your sentence is "share", not "adults". Actually, it's "share of adults", but the word "Share" is what you need to look at, not "adults". Therefore, since that word's singular, so should the word "lack".

"A growing share of adults lacks insurance".

To test this, remove "of adults" from the sentence:

"A growing share lacks insurance". Notice, it also matches the article, "A", which is also singular.

Share is the subject. Therefore 'lacks' is correct (A...share... lacks...). Adults is the subject of the prepositional phrase "of adults".

A growing share of adults lack insurance. You are correct.