Why is their in the following sentence wrong?
The modern American family differs in many significant ways from their nineteenth-century counterpart.
It's wrong because family is singular and their should be used for plurals (apart from singular their, which can't be the case here).
For singular you use its, therefore:
Or you make the subject plural, thus having:
OP's sentence is wrong for most Americans because they normally, esp. in recent decades, treat "the family" as singular, so they would expect "its counterpart", not "their counterpart".
It's wrong for Brits because although it's true we often treat "the family" as plural, if we did that in this particular case we'd obviously expect "The ... family differ", not "differs".
But it's worth pointing out that Brits don't always treat these "group nouns" (family, Parliament, the government, company names, etc.) as plural. In this specific case I think most of us would use the singular anyway, because the family is referenced as a single entity being compared to another single entity (its "collective" counterpart).
It's also worth pointing out that I doubt many native speakers would be happy with...
...which just goes to show that there's no single "logical" approach anyway. I put that example up because you can fix it using singular "they", or simply by saying "the family" is plural anyway (in which case you can legitimately use "plural 'they'"). But if you insist "the family" is singular, and you don't endorse "singular 'they'", you're stuck with the very ungainly form above!
It's not wrong, but I would have worded it like so:
Here everything is singular: family / differs / its / counterpart.
If you substitute the singular they in that sentence, it's equally valid and becomes what you had originally written:
Another way to word it is to make everything plural:
From what I understand, 'their' is also a sign of ownership. From what I further understand, it is part of what you're implying when you say it is THEIR counterpart.
If we were to completely take away the middle part, we'd get this...
Now, you can use 'its', but while it is still an unresolved issue with ALL of the English language, it is one of those accepted things. Singular 'they' does exist and it CAN BE used here. (My earlier argument, for this case just is another piece of evidence for the pro-singular 'they'.)
I'd further like to say that you have listed the noun, it is not nondescript. You refer to a family as a person or as (a) people. That is another reason why 'their' is correct. 'Family', in this case, is being viewed as people, as in plural. (And this last bit was for everyone who thought the sentence was wrong.)
Using their in that sentence is not wrong. It is right for either of two different paths:
Either way, the usage is correct.
Of course, correctness has never stopped people from complaining about things; people will always complain.
It's wrong because their is plural and counterpart, to which their belongs, is singular. That simple. Whether family is singular, plural or both, and whether they can be singular if paired with a non-specific referent (both fruitful sources of disagreement, obviously) have no bearing on the case.
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