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James_Miller comments on [Question] Adoption and twin studies confounders - Less Wrong Discussion
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One confounder for twins is that they were in resource competition when they were in their mom's womb. This environmental effect probably causes twins to appear more different and results in us, when using them to study genetic influence, underestimating the role of genetics.
Wouldn't this happen to both identical and fraternal twins?
Not quite. Identical and fraternal twins have different "intrauterine competition" issues, which are generally worse for identical: identical twins, in the original splitting, may get clumps of cells different in various ways (but fraternal twins, stemming from different eggs, get 100% of their respective egg); identical twins usually share the same placenta which causes a lot of problems & competition, while fraternals get separate placentas; and more obscurely, identicals may share an amniotic sac.
(Every time I read in detail about pregnancy, I can't help but think it's a really freaky and complex process.)
Of course, there are other biases. For example, identical twins aren't actually perfectly genetically identical, as they come with various new mutations and copy-errors and whatnot, so if you assume they are 100% the same, that may bias the estimate downward just like the 'identical womb environment as with fraternal and singles' assumption does, and there's measurement error in IQ scores, which generically leads to underestimates of anything to do with IQ. But there are other biases upward, and I don't know if there's any consensus on what the net is. People who hate hate hate the idea of IQ and there being any genetics there of, such as Shalizi, will certainly bend your ear about problems with the assumptions, but are they engaged in motivated cognition and making mountains of methodological moleholes? Dunno. I'm happy to wait for the GWAS studies. We'll see which emperor has no clothes.
It seems to me that there wouldn't be any selection pressure for identical twins to compete for resources (if someone has the same genome as you, you don't acquire any fitness advantage from competing with them), but I suppose there may be competitive instincts that are selected for in the case of fraternal twins for which there is no shutoff switch in the case of identical twins.
Why would this make them more different? Do you think one twin wins a resource competition and the other loses? Why do you think that?
No, resource competition is an environmental influence that twins share and it makes them similar to each other and different from single births. For example, it suppresses IQ. The most extreme similarity is that it makes them more likely to miscarry, especially boys, but this confounds other measurements.
I don't understand your questions. Why would it not make them more different? The competition introduces another source of variability: what fraction of the resources a particular fetus gets. A singleton has no such randomness, since it just gets 100%, there's no competition.