4 ms·

Why? The twins who were adopted were of the exact same race, gender, age, and appearance at the moment of adoption. People aren't just given a random child when

by emmett 17y ago

Why? The twins who were adopted were of the exact same race, gender, age, and appearance at the moment of adoption. People aren't just given a random child when they adopt, they choose. So you still haven't untangled selection effects entirely.

yummyfajitas 17y ago
The only one of those factors that can't be controlled for very easily is appearance at the moment of adoption. def random_twin_like_pair(children): x = random(children) y = random([c for c in children if (c.race, c.age, c.gender) == (x.race, x.age, x.gender)]) return (x,y)
- emmett 17y ago
  At which point you're down to n of 1-2 per group, because separated-at-birth twin studies commonly contain no more then 30-40 pairs of twins, and your study is fatally flawed. I'm not saying twin studies couldn't be done well, I'm saying in practice most real twin studies are not large enough or well designed enough to actually control for those outcomes. There just aren't enough identical twins separated at birth to allow it to be done well. (That's not to mention further issues like womb environment, which further muddy the issue and are very important) Here's a summary of what's wrong with twin studies in general: http://drbeetle.homestead.com/twins.html http://drbeetle.homestead.com/twins.html
  - yummyfajitas 17y ago
    You don't need to restrict to demographic cross sections. You can take a set of 30-40 twins (of all demographics) and compare them to a set of N>40 randomly selected twin-like pairs (of all demographics). As long as your twins and twin-like pairs have the same demographic mix, you have an adequate sample. This sample will average out demographic variation, but the twin-induced correlations are controlled for. Womb environment can be controlled for by comparing identical to fraternal twins. It may be the case that most twin studies are performed badly. I'm just pointing out that most of the issues you raise are not a real problem for anyone who understands basic statistics (admittedly, this may exclude most psychologists).
    - emmett 17y ago
      Yes, I generally agree that it would be theoretically possible to do a separated at birth identical twin study. In practice, the requirements are so stringent that they've never (to my knowledge) been met. The setup you describe could indeed separate genetic/genetic+environment feedback component from the purely environmental component. Albeit with caveats: - Identical twins given up for adoption and then separated at birth are probably not drawn at random from the population. - You can't draw any conclusions at all about which genetic variance is causing the issue, unless you restrict the demographic cross sections. So it can't possibly give you evidence that one race has a higher average IQ, for example. - The conclusions you draw may be much more about environmental feedback than genetics - maybe with a different environment, the differences will go away. For example, maybe you've found genes that code for susceptibility to iodine deficiency, and your society hasn't discovered iodized salt yet. In that case it's hard to say that you've determined the degree to which intelligence is "genetic" except in the most general sense. In practice, locating identical twins separated at birth is hard. Finding a representative sample of them across demographic mix is harder. Doing it again for fraternal twins to control for womb environment makes it yet more work. I'm not sure how you control for the fact that the identical twins who are given up and then separated at birth are probably a different population from random adoptees, but I don't doubt with sufficient cleverness it could be done. The problem is that real twin studies almost always fail to control all these factors, and failing to control even one factor dooms your entire experiment.
    - tokenadult 17y ago
      As long as your twins and twin-like pairs have the same demographic mix, you have an adequate sample. Proof of this statement? There are mathematical definitions of what size of sample is "adequate" for given effect sizes. (What are the effect sizes of some of the differences in IQ we are talking about here, and how do they compare to error of estimation in the most reliable brands of IQ tests?) It may be the case that most twin studies are performed badly. That would definitely be an important matter to look at.
tokenadult 17y ago
This is correct. After the article was submitted, I went to the University of Minnesota main library (Wilson Library) and Bio-Medical Library to look up monographs on IQ and on human genetics. Adoption agencies, as one widely used genetics textbook notes, have taken care to place separated twins in matched, similar environments, so a study of identical twins reared apart still tends to understand the influence of environment on the development of most human traits.

by emmett 17y ago

yummyfajitas 17y ago

The only one of those factors that can't be controlled for very easily is appearance at the moment of adoption. def random_twin_like_pair(children): x = random(children) y = random([c for c in children if (c.race, c.age, c.gender) == (x.race, x.age, x.gender)]) return (x,y)

emmett 17y ago
At which point you're down to n of 1-2 per group, because separated-at-birth twin studies commonly contain no more then 30-40 pairs of twins, and your study is fatally flawed. I'm not saying twin studies couldn't be done well, I'm saying in practice most real twin studies are not large enough or well designed enough to actually control for those outcomes. There just aren't enough identical twins separated at birth to allow it to be done well. (That's not to mention further issues like womb environment, which further muddy the issue and are very important) Here's a summary of what's wrong with twin studies in general: http://drbeetle.homestead.com/twins.html http://drbeetle.homestead.com/twins.html
- yummyfajitas 17y ago
  You don't need to restrict to demographic cross sections. You can take a set of 30-40 twins (of all demographics) and compare them to a set of N>40 randomly selected twin-like pairs (of all demographics). As long as your twins and twin-like pairs have the same demographic mix, you have an adequate sample. This sample will average out demographic variation, but the twin-induced correlations are controlled for. Womb environment can be controlled for by comparing identical to fraternal twins. It may be the case that most twin studies are performed badly. I'm just pointing out that most of the issues you raise are not a real problem for anyone who understands basic statistics (admittedly, this may exclude most psychologists).
  - emmett 17y ago
    Yes, I generally agree that it would be theoretically possible to do a separated at birth identical twin study. In practice, the requirements are so stringent that they've never (to my knowledge) been met. The setup you describe could indeed separate genetic/genetic+environment feedback component from the purely environmental component. Albeit with caveats: - Identical twins given up for adoption and then separated at birth are probably not drawn at random from the population. - You can't draw any conclusions at all about which genetic variance is causing the issue, unless you restrict the demographic cross sections. So it can't possibly give you evidence that one race has a higher average IQ, for example. - The conclusions you draw may be much more about environmental feedback than genetics - maybe with a different environment, the differences will go away. For example, maybe you've found genes that code for susceptibility to iodine deficiency, and your society hasn't discovered iodized salt yet. In that case it's hard to say that you've determined the degree to which intelligence is "genetic" except in the most general sense. In practice, locating identical twins separated at birth is hard. Finding a representative sample of them across demographic mix is harder. Doing it again for fraternal twins to control for womb environment makes it yet more work. I'm not sure how you control for the fact that the identical twins who are given up and then separated at birth are probably a different population from random adoptees, but I don't doubt with sufficient cleverness it could be done. The problem is that real twin studies almost always fail to control all these factors, and failing to control even one factor dooms your entire experiment.
  - tokenadult 17y ago
    As long as your twins and twin-like pairs have the same demographic mix, you have an adequate sample. Proof of this statement? There are mathematical definitions of what size of sample is "adequate" for given effect sizes. (What are the effect sizes of some of the differences in IQ we are talking about here, and how do they compare to error of estimation in the most reliable brands of IQ tests?) It may be the case that most twin studies are performed badly. That would definitely be an important matter to look at.

tokenadult 17y ago

This is correct. After the article was submitted, I went to the University of Minnesota main library (Wilson Library) and Bio-Medical Library to look up monographs on IQ and on human genetics. Adoption agencies, as one widely used genetics textbook notes, have taken care to place separated twins in matched, similar environments, so a study of identical twins reared apart still tends to understand the influence of environment on the development of most human traits.