There was an interesting question on our sister site ELL:

Suppose a kid asks their mother "Am I your child or Dad's child?": Why are the alternatives in the title grammatically wrong?

  • *You're both of us' child
  • *You're both of our child
  • *You're both of ours child

After all we can have coordinated plurals as the possessors:

  • You're [Mary and John]'s child

You can obviously have:

  • You're [the McDonald]'s child

You can have:

  • You're [the couple who robbed them]'s child

Yes, I know: it's not elegant - but it's grammatical nonetheless.

What reasons are there for the examples in the title being ungrammatical?

This question isn't about how to rephrase the kid's answer. It's about why the examples are ungrammatical.

  • "You are both of us' child" or "You are both of us's child" would appear not to be ungrammatical, although it's certainly awkward and would normally be rephrased.
    – Andrew Leach
    Jan 3 '16 at 21:02
  • @AndrewLeach That's interesting. It feels very wonky to me, not just awkward. I wonder what other people will think ... ? Jan 3 '16 at 21:04
  • @AndrewLeach Out of interest, which one's best for you: "You're both of them's child" or "Your both of their's child"? It's a bit weird because I kind of agree that "us'(s)" is the least bad. But it doesn't follow through for me for the other pronouns. Also without the following noun, "child", I feel that "You're both of ours" is ok, whereas "You're both of us'(s)" is far wonkier. Jan 3 '16 at 21:19
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    @haha Good question too! I don't think it's ungrammatical, but it seems to me that the meaning's different. It would imply that our child possessed something: "Both of our child's friends are coming to the party" for example. Here the both is there because there are two friends, not because there are two of us, I think. :) Jan 3 '16 at 21:50
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    @Araucaria It's not possible. "You're our child" would seem a reasonable alternative. The "our" bit (i.e. us, each of your parents) would be clear in the conversation by deixis.
    – BillJ
    Jan 3 '16 at 21:56

As you note, you can have co-ordinated plurals:

  • You're [Mary and John]'s child
  • You're [the McDonalds'] child
  • You're [the couple who robbed them]'s child

All use a phrase and the Saxon genitive. The last one is grammatical but awkward enough to rephrase as

  • You're the child of the couple who robbed them

This would suggest that any similar structure is grammatical:

  • You're [both of us]'s child

And again, that's awkward enough to rephrase:

  • You're the child of both of us.

The noun phrase is not changed whichever genitive form is used:

  • You're Mary and John's child :: You're the child of Mary and John
  • You're both of us's child :: You're the child of both of us
  • *You're both of our child :: *You're the child of both of our

The forms with our are ungrammatical because although we's or us's becomes our, this doesn't happen with the noun phrase both of us: the possessive has to apply to the whole phrase, not simply the last word. In order to apply it to the whole phrase, it's necessary to use the Saxon genitive just as with the couple who robbed them — that doesn't become their.

Using our actually divorces that possessive from both, so that both refers to the following noun (so child is obviously wrong, because both requires a plural).

  • *Both of our child
  • Both of our children

Because of the ungrammatical nature of the example, it's necessary to change it slightly to produce something which is grammatical, demonstrating that both refers to children:

  • You're both of our children, and we never had any more.
  • Edited: Nice points, nicely presented. Am pondering ... At the moment, my thoughts are that them is necessarily accusative because it's an Object in the couple who robbed them, but us/ours isn't necessarily accusative. It could be genitive. The phrases both of us and both of ours are equally well formed. I still can't tell whether both of us('s) is grammatical in my variety of English - though I'm not saying it isn't in any others. Hmmm ... Jan 3 '16 at 21:55
  • "Both of us" and "Both of ours" are different from each other, and different again from "Both of us's". I hold [obv!] that the last is strictly grammatical, but sufficiently awkward to have been eradicated. Since it is eradicated, it's even more awkward, a vicious or virtuous circle, depending on your point of view.
    – Andrew Leach
    Jan 3 '16 at 21:57
  • +1 Ok, I think you might have something in your answer, but I can't tell. My first problem is that both of ours seems good and I can't see why both of us's chils should use an accusative extrapolating from that. My other problem is that it still seems ungrammatical to me (as in it kicks my natural ear funny bone). But I'm willing to accept that it isn't for you and maybe there's a N/S divide going on, or maybe it's just my idiolect. I still want to know why it's wonky in my variety of English - but I think you have a point, especially about it cliticising on a whole NP, not a noun. Jan 3 '16 at 22:02
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    I don't know about N/S; maybe E/W. I'll have a think about the accusative point with a view to updating the answer, but it's getting late here.
    – Andrew Leach
    Jan 3 '16 at 22:04
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    Actually, I think the both is the problem. Consider "You're both of them's child," which I believe to be strictly grammatical, but so awkward due to them's and its uneducated overtones for "They are" that it has to be changed. That doesn't seem to be the case for "the couple who robbed them". Coo. Interesting.
    – Andrew Leach
    Jan 3 '16 at 22:10

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