-[TODO: link, Cynthia M. Beall, "Two Routes to Functional Adaptation", double-check what "resting ventilation" means]
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-The third part of the book is about genetic influences on class structure! Untangling the true causes of human variation is a really hard technical philosophy problem, but behavioral geneticists have at least gotten started on the problem with their simple _ACE_ model.
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-[p. 212-4: A + C + E model and comparing identical and fraternal twins (different from twins raised apart)]
-[ACE model assumes no assortative mating, which leads to an underestimate of A: because it makes fraternal twins resemble each other for non-environmental reasons]
-[equal environments assumption could be violated]
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-[shared environment is zero for personality]
+The third part of the book is about genetic influences on class structure! Untangling the true causes of human variation is a really hard technical philosophy problem, but behavioral geneticists have at least gotten started with their simple _ACE_ model. It works like this: first, assume (that is, "pretend") that the genetic variation for a trait is _additive_ (if you have the appropriate SNP, you get more of the trait), rather than exhibiting _epistasis_ (where the effects of different loci interfere with each other) or Mendelian _dominance_ (where the presence of just one copy of an allele (of two) determines the phenotype, and it doesn't matter whether you heterozygously have a different allele as your second version of that gene). Then we pretend that we can partition the variance in phenotypes as the sum of the "additive" genetic variance _A_, plus the environmental variance "common" within a family _C_, plus "everything else" (including measurement "error" and the not-shared-within-families "environment") _E_. Briefly (albeit at the risk of being _cliché_): nature, nurture, and _noise_.