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.
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.