Confounding factors aside, people who eat according to the rules of a traditional food culture are generally much healthier than people eating a contemporary Western diet. This goes for the Japanese and other Asian diets as well as the traditional diets of Mexico, India, and the Mediterranean region, including France, Italy, and Greece. There may be exceptions to this rule—you do have to wonder about the Eastern European Jewish diet of my ancestors. Though who knows?
-Michael Pollan, In Defense Of Food
But a mutation load/purifying selection balance was one of the more elegant theories for why intelligence variants haven’t reached fixation given their apparent obvious utility, so this raises that question even more. Why are there genetic intelligence differences? Especially when the phenome studies (only some of which Yvain cited) show pervasive genetic overlap between genes which increase intelligence and genes which increase all sorts of other desirable traits like less schizophrenia risk? Right now I think probably the best theory is a resources or developmental one: a high quality brain, and body, are extremely metabolically demanding (did you know that in childhood, your brain and body have to take turns growing, because it’s metabolically impossible to do both?), and so any pro-intelligence variants runs into the risk of increasing vulnerability to famine and infection and injury; so you get selection for intelligence variants only up to the point where diminishing returns kick in hard, and then it’s better to have a more robust immune system or to stop growing early on and adopt more of a r-selected strategy, and then in the modern context where calories are abundant, education & intellectual pursuits apparently have been consistently dysgenic, so that eliminates any recent chance for driving pro-intelligence variants to fixation or even increasing their frequencies noticeably.
Unsourced but commonly known fact: We observe higher IQ consistently in Ashkenazi Jews (just Ashkenazim, not Sephardim or any other strain, nor their non-Jewish neighbors).
Crackpot theory: the traditional diet of Ashkenazi Jews is very rich because it reflects higher caloric needs than gentiles. Probably wrong; it’s not that different from the traditional diet of my Catholic ancestors from the same general area, and the ‘blubber’ theory probably explains this better.
(I’ll be back to Jane Jacobs soonish; I have a couple posts drafted but may read through the whole book before posting this time, since I’m chunking chapters together.)