I had read something like this, and previously asked a question about "cocker spaniel eyes" , from which I've got some very helpful answers. But a new question was aroused then, as what Mr. PeterG said:
I thought that point which hadn't been noticed was quite interesting. After thinking over that, I thought I would see it this way:
"his brown wavy hair seemed to have a movement of its own" can be understood in a way that his hair, as a whole mass, goes in a same direction of its own. But I have a different understanding that the author actually uses "their" to mean each strand of his hair goes in its own direction, that is to say, the boy has rough or Afro hair.
"his brown wavy hair" is deemed to be a mass noun with a singular form, whereas "have a movement of their own" has to lay stress on each individual hair's own movement, and in this case, the author has no way to avoid making "hair" and "their" contradict each other according to strict singular/plural rules. In a word, I don't think it's a mistake. Here, logic should be given greater priority over some rigid grammar rules.
It seems that Mr. PeterG doesn't agree to regarding it as a right sentence. I am not a native speaker, I am somewhat ready to accept his argument, but I still need more evidence to be convinced.
Is it grammatically wrong to use ”their“ in this case to refer to hair？
Best Answer
Why it’s wrong
Yes, it’s grammatically wrong. But your way of thinking about English grammar is right in a very profound way: wondering if singular and plural could be exploited to suggest some subtle distinction.
Here’s why it’s wrong. The pronoun their calls for a plural antecedent. Hair is singular. Also, movement is singular, so it has to be grammatically tied to an individual hair. If you tie it to the mass of hairs all together, then it suggests that the mass of hairs all together (what we normally call “hair” with no determiner) has one movement as a whole.
How to do it right
There are ways to use singular vs. plural to indicate what you have in mind. Here’s how you’d do it:
The singular word each gives the singular movement something to attach to, and hairs is plural. This makes it clear that the sentence is talking about many movements, not just one.
You could also indicate separate movement of each strand of hair like this:
You are right that logic often trumps rigid grammar rules, leading a reader to interpret a sentence reasonably when too-strict application of grammatical regularity would lead to clumsiness. But since the language provides a straightforward way to indicate the intended meaning in this case, there’s no pressure to bend grammar.
The inevitable complexity
Their can take a singular antecedent when it stands for a person and you’re trying to avoid indicating “their” gender. However, to many people's ears, this usage sounds sloppy or ungrammatical, or at best informal, because their calls for a plural antecedent. There is currently something of a war going on in the language right now, to allow their to refer to a singular person as antecedent in order to avoid sexist language. Perhaps after that war is won, their will broaden to allow singular antecedents of all kinds, but today such a development is beyond the horizon.