1) Puche restored IGF-I circulating levels, which normally decline in serum with age, in aging rats.

2) Puche restored IGF-I circulating levels in aging rats, which normally decline in serum with age.

In most cases, "which" is used to indicate the word closely before it like "soldiers opened fire on a car which failed to stop at an army checkpoint". But as for the two sentences above, I am not sure which one is correct?

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    There is a difference: in your "soldiers" example, the relative clause is of the integrated (defining) kind. It refers only to "car" - it identifies which car is being talked about. But in your first two examples, the R/C is supplementary (non-defining), and this kind of relative can refer to virtually anything. In your first example, "which" refers to "IGF-I circulating levels", and in the second to "IGF-I circulating levels in aging rats". – BillJ Apr 23 '17 at 7:02

It refers to the phrases, not words. In #1, it refers to "IGF-I circulating levels". In #2, it refers to "IGF-I circulating levels in aging rats".

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    Technically it's ambiguous and requires care, but in context the interpretation you give is fairly clear, and IMO sufficiently so. – Lightness Races in Orbit Apr 23 '17 at 15:31

Which is a relative pronoun that can refer:

  • to a preceding word:

    I'll have the book which is at the top of the pile.

In the preceding example it introduces a defining relative clause, that is to say the relative clause is essential to understand what we are talking about. That could also be used in that sentence.

  • or to a whole sentence or clause:

    I always get up at six in the morning, which is really difficult.

    Puche restored IGF-I circulating levels in aging rats, which normally decline in serum with age.

In these last examples which introduces a non-defining relative clause, it gives extra information and is not necessary in order to understand what we are talking about. In that case it could not be replaced by that and we use commas around the non-defining clause (or ends with a period if it ends the sentence).

Your first sentence sounds awkward because you split "circulating levels" and "in aging rats" which should really stand as one block because - as far as I can understand it - Puche restored IGF-I circulating levels in aging rats and not in any other kind of rats or in other animals.

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In the first example, "which" can only refer to "IGF-I circulating levels". "which " can never refer to something after the "which" clause.

The second example is confusing for the reason you stated in the question. Unless you are writing for an audience which already knows about the topic, it is not clear what the "which" refers to.

I'm not a biologist, so I don't know if "aging rats decline in serum" or if "IGF-I circulating levels decline in serum with age", and the second example doesn't answer that question!

Personally I would restructure the second sentence as something like

IGF-I circulating levels in serum normally decline with age; Puche restored the (serum? IGF-I?) levels in aging rats.

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  • Semantics does make the first of the two readings you propose very unlikely, though: “Aging rats decline with age” is tautological to the point of bizarreness. Grammatically, which could refer to either the full NP or just the head; semantically, only one seems realistic in this particular case. – Janus Bahs Jacquet Apr 23 '17 at 12:47

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