These are two separate links for two separate claims. âTwin studies are flawed in methodology.â and âTwins, even identical twins, simply do not have exactly the same DNA.â, both of which are true. The confidence in the proposed HBD conclusions is simply not warranted by the evidence.
Oops! Sorry, I only discovered the second link; but before writing my comment, I looked up the first myself.
Iâm not a biologist and will probably defer to any biologist entering this thread and commenting on the twin studies.
Twins (mostly, as the linked study shows) do not have exactly the same DNA. But it doesnât seem to be relevant. The relevant assumption is that thereâs almost no difference between the DNAs of âidenticalâ twins and a large difference between the DNAs of non-identical reared-together twins, which is true despite a couple of random mutations per 6 billion letters.
The next two linked articles are paywalled. Is there somewhere to read them?
The third is a review of a short book, available after a sign-up, and it says that âsome studies on twins are good, some badâ, and the author feels, but âdoesnât actually knowâ that the reviewed one is good. The reviewed book performed a study on twins and noticed there isnât much of a difference between the correlation of the similarity of many personality traits with whether people are identical twins, and concluded that, since youâd expect to see a difference if the traits have different degree of heritability, many personality traits are results of the environment.
How is this an evidence that twin studies are flawed and shouldnât be used? If thatâs a correct study, itâs just evidence that personality traits are mostly formed by environment (which is something I already believe and have believed for the most of my life), but, e.g., why would this be relevant for a discussion of whether or not some disease has a genetic component to it, when a twin study shows that there is?
Itâs important to carefully compare the numbers; but obviously there are things that identical twins have in common more often then non-identical twins, because these things are heritable at to larger or lesser degree; like hair color or height.
Of course, any study makes some underrepresentation of humanity. But if your study is about the degree of heredity of something and not about twins, why would this matter? If thereâs a difference between adopted identical and non-identical twins thatâs better explained by genetics (e.g., non-identical twins would have a different height more often), why does it matter how well they represent twins in general? Unless youâre studying how likely people are to be adopted, I donât understand the claim.
The last link is paywalled, but again, why would this affect the difference between identical twins and non-identical twins? Until a year ago, I kept secret that Iâm bi and wouldâve kept it secret from scientists; but I donât think this kind of thing affects conclusions youâd make if identical twins answered identically to some question more often than non-identical twins (e.g., imagine a society where people with green eyes are persecuted and a lot of them use contact lenses. Some would still say the truth, in confidence, to scientists; and the number of identical twins telling the same answer would be greater than the amount of non-identical twins telling the same answer, and the scientists will correctly infer this to be evidence for the heritability of eye color, even though a lot of twins would lie about their eye color).
So while itâs possible to just compare full DNAs and account for lots of different factors (all sorts of various environmental conditions that might be different between the subjects of the study) to find out whether DNA correlates with eye color, itâs much easier to do a twin study, and a strong correlation there will be a strong evidence
Itâs fine. Studies donât just use identical twins but twins in general. You are equating my two claims and attacking claims that I havenât even made, I never talked about âwhether or not some disease has a genetic component to it, when a twin study shows that there is?â. I made a claim that twins, even identical twins, donât share exactly the same DNA and provided a link to an article that gave more information, and I made a second claim that twin studies were flawed and provided that claim with a link to an article with more information about that. All this stuff about that it canât help us find diseases or that twin studies âshouldnât be usedâ are claims I never made.
EDIT:
Iâm not a biologist and will probably defer to any biologist entering this thread
For the record my study has some biostatistics, but it isnât my strongest field and Iâm mostly leaning on stuff my professors have explained:
I will also probably defer to a biologist/âbiostatistician.
These are two separate links for two separate claims. âTwin studies are flawed in methodology.â and âTwins, even identical twins, simply do not have exactly the same DNA.â, both of which are true. The confidence in the proposed HBD conclusions is simply not warranted by the evidence.
Many twin studies have the assumption that they share 100% of their DNA (which is false) andthat they share the exact same environment (which is also false). This leads to underestimating environmental factors and underestimating non-genetic biological factors.
Furthermore, separated twin pairs, identical or fraternal, are generally separated by adoption. This makes them unrepresentative of twins as a whole and there can be some issues of undetected behaviors in the case of behaviors that many people keep secret presently or in their earlier lives.
Oops! Sorry, I only discovered the second link; but before writing my comment, I looked up the first myself.
Iâm not a biologist and will probably defer to any biologist entering this thread and commenting on the twin studies.
Twins (mostly, as the linked study shows) do not have exactly the same DNA. But it doesnât seem to be relevant. The relevant assumption is that thereâs almost no difference between the DNAs of âidenticalâ twins and a large difference between the DNAs of non-identical reared-together twins, which is true despite a couple of random mutations per 6 billion letters.
The next two linked articles are paywalled. Is there somewhere to read them?
The third is a review of a short book, available after a sign-up, and it says that âsome studies on twins are good, some badâ, and the author feels, but âdoesnât actually knowâ that the reviewed one is good. The reviewed book performed a study on twins and noticed there isnât much of a difference between the correlation of the similarity of many personality traits with whether people are identical twins, and concluded that, since youâd expect to see a difference if the traits have different degree of heritability, many personality traits are results of the environment.
How is this an evidence that twin studies are flawed and shouldnât be used? If thatâs a correct study, itâs just evidence that personality traits are mostly formed by environment (which is something I already believe and have believed for the most of my life), but, e.g., why would this be relevant for a discussion of whether or not some disease has a genetic component to it, when a twin study shows that there is?
Itâs important to carefully compare the numbers; but obviously there are things that identical twins have in common more often then non-identical twins, because these things are heritable at to larger or lesser degree; like hair color or height.
Of course, any study makes some underrepresentation of humanity. But if your study is about the degree of heredity of something and not about twins, why would this matter? If thereâs a difference between adopted identical and non-identical twins thatâs better explained by genetics (e.g., non-identical twins would have a different height more often), why does it matter how well they represent twins in general? Unless youâre studying how likely people are to be adopted, I donât understand the claim.
The last link is paywalled, but again, why would this affect the difference between identical twins and non-identical twins? Until a year ago, I kept secret that Iâm bi and wouldâve kept it secret from scientists; but I donât think this kind of thing affects conclusions youâd make if identical twins answered identically to some question more often than non-identical twins (e.g., imagine a society where people with green eyes are persecuted and a lot of them use contact lenses. Some would still say the truth, in confidence, to scientists; and the number of identical twins telling the same answer would be greater than the amount of non-identical twins telling the same answer, and the scientists will correctly infer this to be evidence for the heritability of eye color, even though a lot of twins would lie about their eye color).
So while itâs possible to just compare full DNAs and account for lots of different factors (all sorts of various environmental conditions that might be different between the subjects of the study) to find out whether DNA correlates with eye color, itâs much easier to do a twin study, and a strong correlation there will be a strong evidence
Itâs fine.
Studies donât just use identical twins but twins in general. You are equating my two claims and attacking claims that I havenât even made, I never talked about âwhether or not some disease has a genetic component to it, when a twin study shows that there is?â. I made a claim that twins, even identical twins, donât share exactly the same DNA and provided a link to an article that gave more information, and I made a second claim that twin studies were flawed and provided that claim with a link to an article with more information about that. All this stuff about that it canât help us find diseases or that twin studies âshouldnât be usedâ are claims I never made.
EDIT:
For the record my study has some biostatistics, but it isnât my strongest field and Iâm mostly leaning on stuff my professors have explained:
I will also probably defer to a biologist/âbiostatistician.