Is two-thirds plural?

Is 2/3 always, sometimes or never plural?

E.g.

1a) 2/3 of the pizza were eaten.
1b) 2/3 of the pizza was eaten.

2a) 2/3 of the visitors were men.
2b) 2/3 of the visitors was men.

I feel that example 1 could go either way but example 2b sounds very wrong.

Is there a rule for this?

Solution 1:

It depends on whether two-thirds (or any similar proportion) is regarded as a measure of amount or of number. In (1), the emphasis is likely to be on the amount of pizza eaten, and not on the number of individual thirds, so (b) would be appropriate. In contrast, in (2) the emphasis is on the number of visitors who were men, so plural concord, as in (a), is required.

Solution 2:

Yes and no. Yes, when you're talking about multiples of fractions eg. two thirds they are plural. But when you're talking about a portion of a single item, that item is singular.

1). One third of the pizza was eaten. (part of one pizza was eaten)

Two thirds of the pizza was eaten. (part of one pizza was eaten)

The subject is pizza - singular. If you have one third of a pizza, and I have one third of that same pizza we have two thirds (plural) of one pizza (singular).

Of course this changes when the subject becomes plural:

One third of two pizzas were eaten. (part of two pizzas were eaten)

2). One third of the visitors were men. (multiple visitors were men)

Two thirds of the visitors were men. (multiple visitors were men)

The subject in this case - men - is plural.

The sentence doesn't make sense if the subject is singular (Two thirds of the visitors were man). I'll just change the meaning slightly so it works:

One third of the man was on fire. - Singular Two thirds of the man was on fire. - Still singular

Clear as mud.

Solution 3:

It depends on what the fraction is of. If you are referring to two-thirds of an uncountable noun ("two thirds of love is understanding") then any further reference to the "two thirds" should be singular. If you are referring to two thirds of some finite (countable) quantity, such as "two-thirds of people in the U.S. are brunettes", then that "two-thirds" reference should be plural.