Let me choose date of birth as the noun because each person exactly has only one.

Which is the correct sentence? A or B?

A: They write their date of birth on the given form.

B: They write their dates of birth on the given form.

Note: Here I assume that each people gets a separate piece of form so a confusion whether I have to write as follows in case there are more than one forms are given to each of them can be avoided.

A': They write their date of birth on the given forms.

B': They write their dates of birth on the given forms.

  • 2
    Both are correct usage although A is when you are using 'their' for one person of unspecified gender, B is when you are referring to more than one person. However this is broadly a duplicate of Grammatical number agreement in a complex phrase using singular "they" – Ali Beadle Mar 25 '17 at 7:33
  • @AliBeadle; I don't think your proposed duplicate works: the issue in this question is not they and whether it's singular or plural, but about whether multiple people have a date of birth (one each) or multiple dates of birth. – JavaLatte Mar 25 '17 at 8:28
  • @JavaLatte I see, yes. So B or B' are correct given the assumption that they are talking about multiple people, but with the caveat that A would be correct if they wanted to talk about one person but not specify gender. – Ali Beadle Mar 25 '17 at 8:35
  • I say it should be singular, it should be in a sense Each of them." Every person of "they" writes their one date of birth They can't have more than one. – SovereignSun Mar 25 '17 at 14:47
  • If you give more than one form to every "they" then A' is correct. Else A is correct. – SovereignSun Mar 25 '17 at 14:50

Their X just means X belongs to multiple people.

If each of the "their" has an X, then you are talking about multiple Xs, and should use the plural form of X.

If everyone in the "their" group is sharing a single X, you would use a singular X.

An each can emphasize the "everyone has their own X" and can override this.

They each walked with their girlfriends.

They each walked with their girlfriend.

The second sentence won't raise any eyebrows even though technically it means something unusual.

This could raise eyebrows though:

They walked with their girlfriend.

  • "Both of them took their girlfriend to dinner". In my opinion, there's no real difference between singular or plural. Technically, using the singular allows for the implication that [they] share the same [girlfriend], but this doesn't make the use of the singular form particularly incorrect. – Flater May 11 '17 at 7:32
  • When you say both or even them two that implies each. And you're right, even without it the mature listener would likely make the sensible conclusion irrespective of the details. But I just imagine some smart-ass may still say something if you said "They took their girlfriend to dinner" even though it is unlikely you mean shared girlfriend. – LawrenceC May 11 '17 at 14:40
  • "Jack and John both kissed their mother goodbye". This sentence is correct, regardless of whether Jack and John are siblings or not. It seemingly implies a shared mother, but not necessarily. The sentence merely highlights that Jack and John took the same action, not kissed the exact same person. I would still prefer to use plural to avoid ambiguity, but I think either would be correct, grammatically speaking. – Flater May 11 '17 at 14:43
  • @Flater an similarly, technically using the plural could imply they are polygamous, each of them having several wives... – odalet Nov 28 '20 at 15:48

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