An ideal study of IQ and environment might be as follows:

Find 300 Bushmen babies being raised as hunter-gatherers and randomly assign them to three groups of 100 Bushmen each:

Group 1:  Gets sent to the United States where they are raised by billionaire Ivy League PhDs and the full-blooded children of these adopted Bushmen are also raised by billionaire Ivy League PhDs.

Group 2:  Remains in their hunter-gatherer environment, but gets weekly visits from doctors and nutritionists  to make sure they, and their babies have the exact same First World medical care, health and nutrition as Group 1, but these health professionals are not allowed to speak to them in English or explicitly educate them in anyway; their only role to make sure the Bushmen reach their biological potential which means doing regular health checkups and supplementing any nutritional deficiencies, especially in pregnant women.  If the health professionals do their job, we’d expect the second generation of group 2 to have the same birth weight, infant head circumference, adult height, and perhaps adult MRI brain size as group 1’s second generation.

Group 3:  Remains in their hunter-gather environment with no intervention at all.

Several decades later, the children of all three groups would be administered the Wechsler intelligence scales (in English for group 1, translated into a Khoe language for groups 2 and 3, though for the Vocabulary subtest they would still have to define English words, none of which they would have heard before, but they would define them in their native Khoe)

If this were done, I would expect the subtests of the Wechsler could be divided into the following categories:

Type 1:  subtests where the group 1 > group 2 gap far exceeds the group 2 > group 3 gap.  These would likely be subtests like Information and Vocabulary which requires exposure to Western culture which groups 2 and 3 lacked.

Type 2:  subtests where the group 2 > group 3 gap far exceeds the group 1 > group 2 gap.  These would be subtests where exposure to Western culture and education matters much less than the physical development of the brain.  These would likely include some of the hard-core Wechsler performance subtests where you have to use your hands to quickly fit objects together in a spatially competent way.

Type 3:  subtests where all three groups would score relatively equal.  These are subtests where neither the cultural nor biological environment matters much unless it’s pathological.  Skeptics would deny type 3 tests are even possible, but perhaps some of the Wechsler auditory short-term memory subtests might be type 3.

We don’t have to give the three subtest types names, but it’s tempting to use adjectives like crystallized, achievement, and culturally loaded to describe type 1 subtests, and fluid, aptitude and culture reduced to describe types 2 and 3.  The difference between 2 and 3 being that the former show more phenotypic plasticity, but for biological, not cultural reasons.

The Raven Progressive Matrices is a test which showed enormous phenotypic plasticity over the 20th century (the Flynn effect) even though it was intended to be culture reduced.  This can be partly explained by the fact that Flynn effect is partly biological (Richard Lynn noted that improved nutrition has increased brain size since WWI) and by the fact that the Raven is partly cultural, as James Flynn has argued.

If even the Raven is culturally biased, is a truly culture fair psychometric test even possible?  If we define culture fair as tests where group 1 and group 2, but not necessarily group 3, score equally, these might be possible, but I think the reason the Raven failed was a) it’s boring nature made it too sensitive to test motivation which is a culturally sensitive variable, and b) as James Flynn implied, it relied too much on hypothetical thinking:  people in less modern cultures only apply their intelligence to clearly defined practical looking problems with tangible solutions.

Some of the Wechsler auditory short-term memory subtests or hands-on spatial subtests might come a lot closer to culture fair than the Raven did.

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