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I have the following sentence, and I have a question about it. I’m wondering if there’s something so simple that I’m just overlooking it or whether there is some super-sophisticated tenet of grammar that I’ve not ever learned before.

Two other studies with follow-up times of five or six years revealed similar findings [104, 105], which suggests that physical activity can delay onset of dementia by elevation of the threshold to be exceeded before pathology is exhibited.

Question: I know that singular suggests is the right verb by the sound of it, not plural suggest despite the two studies, but what is the technical reason?

Try this experiment.

First, simplify the sentence:

They revealed similar findings, which suggests that . . .

Now, look at the following paraphrases:

✔ They revealed similar findings. These findings suggest that . . .

✔ They revealed similar findings. This fact suggests that . . .

So what is the referent for the which? Is it the entire clause (singular) or is it findings?

What happens if the second part of the sentence is rephrased to make it essential information by removing the comma?

✔ They revealed similar findings that suggest . . .

✔ They revealed similar findings which suggest . . .

✘ They revealed similar findings that suggests . . .

✘ They revealed similar findings which suggests . . .

Here, the pronoun can only be referencing the findings.

Let's look at two different sentences with dependent clauses that provide nonessential information:

They ate apples, which were red.

They ate apples, which satisfied their hunger.

In the first sentence, which references the apples. In the second sentence, which references the entire clause.

We only know which is correct because of context. It makes no sense to say that the act of eating apples can be red, nor does it make sense to say that apples (literally sitting on their own, without being eaten), could satisfy hunger. (It's assumed that when you say apples will satisfy hunger, you mean eating them—not just, for example, looking at them.)

Returning to the original sentence, what happens if we change it?

They revealed similar findings, which suggest . . .

Assuming that subject-verb agreement is correct here, then which is referring to the findings, not to the entire clause.

I would argue that this is an understandable and grammatical sentence if we assume that the reference is the findings. To make it clearer, most people would probably rephrase it slightly:

They revealed similar findings, each of which suggesting . . .

But while the use of each of before which and suggesting rather than suggest makes the meaning clearer, it's not essential.

Finally, looking at the complete original sentence:

✔ Two other studies with follow-up times of five or six years revealed similar findings [104, 105], which suggests that physical activity can delay onset of dementia by elevation of the threshold to be exceeded before pathology is exhibited.

✔ Two other studies with follow-up times of five or six years revealed similar findings [104, 105], which suggest that physical activity can delay onset of dementia by elevation of the threshold to be exceeded before pathology is exhibited.

In short, strictly speaking, it's ambiguous.

If which is supposed to apply to the clause, then use suggests; if which is supposed to apply to findings, then use suggest.

Assuming that a typo hasn't been made, and that the use of suggest or suggests is informed and deliberate, then its singular or plural form indicates what it's referring to.

In common use, however, most people would think that suggest sounds a little strange, and rephrase the sentence if they did mean which to refer to the findings:

Two other studies with follow-up times of five or six years revealed similar findings [104, 105], each of which suggesting that physical activity can delay onset of dementia by elevation of the threshold to be exceeded before pathology is exhibited.

Note: It's even possible that which refers to the two studies, not to the findings. If so, both of which can be used. Or additional rephrasing could be made.