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I think the boldfaced sentence should be changed to "which mean they share the same genetic material with each other". I wondered whether what I understand is right.

The other thing that's amazing about it is that when you look at the question of genetics, they've done various studies of identical twins, that means they share half of their genetic material with each other, and it turns out that you cannot predict their leadership capacity beyond about one-third in terms of their genetic structure. So in that sense, there is no gene for leadership. -- from a video lecture on leadership

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  • "half of the same genetic material" seems very different from "the same genetic material". Do you think it should be changed because of grammar, or because it reads better your way, or to be more scientifically correct?
    – stangdon
    Dec 30 '21 at 14:21
  • 2
    The entire preceding phrase "means" singular, not plural. It's not: identical twins mean [something].
    – Lambie
    Dec 30 '21 at 14:47
  • I messed up on the phrasing of my comment, but my point stands: "half of their" is not the same as "the same".
    – stangdon
    Dec 30 '21 at 14:50
  • For one thing, it is more scientifically correct, and the other is that grammatically, 'means' should be changed to 'mean' since 'that' refers to 'identical twins' and that 'that' should be changed to 'which' since it is a nonrestrictive relative clause. Dec 30 '21 at 15:00
  • Identical twins, of course, share all of their genetic material. (Except in unusual circumstances.) Ordinary siblings share half of their genetic material. (Again, except in unusual circumstances.)
    – Dan
    Dec 30 '21 at 21:04
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No, "means" is correct — although the sentence is poorly written on the whole, and especially poorly punctuated around that area. I assume it's transcribed from oral conversation.*

There are a few different senses of the word "mean". Here are three relevant ones.

  1. To mean [definition]. e.g. "Isolate means cause to be alone." In this case the subject must be a word, a term, or a phrase.

  2. To mean [consequence]. e.g. "The litmus paper turning red means the solution is acidic." In this case the subject is some fact or action or cause.

  3. To mean [intended message]. e.g. "Hadn't you better lay off the chocolates?" — "What do you mean?" — "I mean you've put on a lot of weight over the holiday." This is the broadest one, and while the subject can be a person, it could also be a book or a song, for instance.

There are more, but that's enough to look at this sentence.

First of all, it's clear that "they share half of their genetic material" is either (1) or (2). The identical twins are not trying to convey a message (3) — they haven't said anything and are not agents here at all. The phrase is given either as a definition of "identical twins" (1) or as an explanation of the relevance for the study (2).

Once we understand this, we see that the only plural subject, "twins", is ruled out. The real subject is either "the term identical twins" or "the fact that they're identical twins".

This kind of implicit subject is extremely common and expected. However, the wording and punctuation of the sentence do not help you to parse it correctly. Here's a better wording for (2):

...they've done various studies of identical twins — which means they share half of their genetic material with each other — and it turns out...

And here's a better wording for (1):

...they've done various studies of identical twins (people who share half of their genetic material with each other), and it turns out...


* In the original oral conversation, I would expect a pause after "identical twins". The speaker then looks at the interviewer, or at the audience, and, raising their eyebrows or index finger meaningfully, adds the parenthetical "That means they share half of their genetic material" and then resumes; this would be meaning (2).

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"Identical twins" is a noun phrase. Although 'twins' is a plural, your example sentence is not referring to a specific set of twins, or even twins in general. The statement is about what that singular phrase means, so 'means' is correct in this case.

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I think this long sentence should be broken down even more to make it easier to comprehend. Like this:

The other thing that's amazing about it is that when you look at the question of genetics, they've done various studies of identical twins. They share half of their genetic material with each other, and it turns out that you cannot predict their leadership capacity beyond about one-third in terms of their genetic structure. So in that sense, there is no gene for leadership.

However, it's hard to evaluate without the context around that little paragraph. Here you can read more about splitting sentences to make them more comprehensive (also goes over the purpose and use of the word "which" as a nonrestrictive element in a sentence).

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