> I don't see any investigation of whether IQ can be altered through nurture (which I think it can)
IIRC, one thing that we have studied pretty thoroughly, is that IQ — that is, the number IQ tests spit out — goes up when people are under the effects of stimulant drugs during the test. (Even neurotypical people, whose executive functioning is just fine without such drugs.)
This is one major reason that IQ is considered a separate thing from the g factor.
Essentially, g factor is "trait intelligence", while IQ is "state intelligence."
While IQ might be altered by nurture/environment/situation, g factor cannot be, by definition. g factor is supposed to be what you get down to when you remove all the state-intelligence confounders. That's why it's a more interesting number to study.
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To couch IQ vs. g factor in a cute analogy: think of ability scores in Dungeons & Dragons, vs. savings throws in that same game.
g factor is essentially your INT ability score. It's defined at character creation, and it's effectively static (for regular humans; the only things that alter it in the game are supernatural.)
But you don't use your INT ability-score directly for anything in the game. Instead, you use your INT modifier — which has your INT ability-score as its base value, but then aggregates on top all other temporary or semi-permanent bonuses/maluses from magical effects, equipment, etc.
If you do a number of "saving throws vs. INT" at a calibrated difficulty level, and average them together, what you'll get is a measure of your character's current INT modifier, not their INT ability score. Their IQ, not their g factor.
(The analogy is surprisingly even tighter than this: people who go into INT-based classes get a proficiency bonus on their INT modifier, somewhat like people who do lateral-thinking puzzles all day get a proficiency bonus on IQ tests. And there are situational modifiers to difficulty, just like there are confounders in the questions to an IQ test.)