+[TODO: link, Cynthia M. Beall, "Two Routes to Functional Adaptation", double-check what "resting ventilation" means]
+
+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 genetic variation for a trait is _additive_ (if you have the appropriate SNP, you get more of the trait), rather than exhibiting epistasis () or Mendelian dominance ().