“fraction of...”: singular or plural?

Just noticed this, which is an interesting parallel to what we've been discussing elsewhere (the thread on team names) - For a start I would prefer "What fraction" to "Which fraction", but the real question is about verb agreement. We're back to notional agreement vs formal agreement - "A large number of people have recently debated this point" or "A large number of people has recently debated this point"? Me, I go for plural.

Google Search:
"What fraction of the fruit are" - 35
"What fraction of the fruit is" - 15
Note that "Which fraction" gets two hits, apart from this thread.

At Google Books, however, it's 7 to 4. But those 4 do include one from the School Mathematics Project at Chicago University.

'Is' may be grammatically more 'correct', but I find 'are' more natural. Perhaps the problem is that there are two possible full answers to "What fraction of the fruit is/are oranges?" - "Oranges are one third of the fruit" (not "is") or "The fraction is one third of the fruit" (not "are") . My first reaction would be to answer "Oranges are ..."This is because the question sounds to me more like "What fraction of the fruit do oranges comprise?" rather than "What fraction of the fruit consists of oranges?"

It's interesting to see what happens in normal statements (rather than strangely worded questions). I looked up the first 50 entries for "only a small fraction of" at Google Books. There were nine examples where it was the subject and related to a countable noun, and where the verb could be distinguished as singular or plural. The result was 7 plural verbs to only 2 singular ones:

"only a small fraction of the transmitted electrons pass through the objective aperture" (pl)
"only a small fraction of RyRs are activated" (pl)
"Only a small fraction of these cells, however, are able to create a micrometastasis" (pl)
"only a small fraction of the planetesimals originally in the vicinity of Neptune was scattered outward: " (sg)
"If only a small fraction of girls are biologically oriented to market rather than household activities" (pl)
"Only a small fraction of natural deaths receives a forensic investigation" (sg)
"Historically only a small fraction of the benefits from innovations have been captured by the innovator" (pl)
"Yet only a small fraction of America's law enforcement officers have been trained to recognize and stop this serious crime" (pl)
"only a small fraction of auto engines that are not functioning optimally produce the majority of pollutants" (pl)

It's a genuinely interesting question, this. It seems to me that in wh-questions with the verb "be" plus a noun, the verb is generally governed by by the noun that follows it: perhaps because it's a subject complement rather than a direct object, it's seen as the subject.

"What are oranges?" (oranges are what?)
"What kind of fruit are oranges?" (oranges are what kind of fruit?)
"What fraction of the fruit are oranges?" (oranges are what fraction of the fruit?)

But judging by searching the Internet, this last type of question is pretty rare. Similar constructions would be (with Google Search figures):

"What percentage of the population are women?" - 9950 ("is" - 5) - women are what percentage
"What proportion of the population are geniuses? "- 7 ("is" - 0) - geniuses are what proportion

When verbs other than "be" are used, it varies, depending on what is seen as the subject:

"What percentage of the population have green eyes?" - 1120 (has - 6680) - "percentage" is the subject of the verb "have"

"What family of fruit do oranges belong to?" - "oranges" are the subject of the verb "belong"

When adjectives are involved after "be", it seems to depend on the noun just before "be"

"What percentage of the fruit is rotten?" (fruit - uncountable)
"What percentage of the people are unemployed?" (people - plural)

Googling "what fraction of the fruit" came up with 61 results. I discounted 3 as being ungrammatical or this thread, 1 was followed by an adjective, 6 were followed by verbs other then "be", and in 6 "fruit" was followed by another word such as "salad" or "slices". That left 42 where "be" was followed by a plural, but of these, 5 were in verb forms which do not differentiate between singular and plural. Of the rest:

"What fraction of the fruit (below/in the bowl) are/were (the) oranges/apples/pears etc?" - 26
"What fraction of the fruit (in the bowl/bag/Dylan ate) is oranges/apples/pears? " - 11

So it seems that, like me, people generally prefer the plural. The singular sounds very weird to me. I have to say, though, that the picture is rather different at Google Books, where singular outnumbers plural 12 - 6. And a respectable publisher can be found for each - Barrons Psat/Nmsqt practice tests- "is", Everyday Mathematics, University of Chicago School Mathematics Project -"are".

There were a couple in Google Search I thought were rather less grammatically ambiguous than "What fraction of the fruit are/is" - one where fraction really is the subject, and one where it is the object:

"What fraction of the fruit salad was made up of strawberries"
"What fraction of the fruit bar did each child get?"