Gone unmentioned is that the recent GWAS study of IQ undermines Flynn and Dickens’ often cited model of IQ, according to which differences within and between population are quite malleable. As Flynn explains, the model is based on the idea that high heritability estimates are an illusionary product of kinship studies. Accordingly, the high estimates reflect undetected gene-environment correlations, not large additive genetic differences. Here’s a 2008 discussion of the model by Flynn:
Seven years ago, William Dickens of the Brookings Institution, decided to do some modeling of his own and asked my help in applying it to real-world situations (Dickens & Flynn, 2001a; 2001b). We believe that it solves the identical twins paradox without positing a Factor X. It makes an assumption that may seem commonplace but which has profound implications, namely: that those who have an advantage for a particular trait will become matched with superior environments for that trait.
Recall studies of identical twins separated at birth and reared by different families. When they grow up, they are very similar and this is supposed to be due solely to the fact that they have identical genes. But for that to be true, they must not be atypically similar in environment, indeed, the assumption is that they have no more environment in common than randomly selected individuals. To show how unlikely this is, let us look at the life history of a pair of identical twins.
John and Joe are separated at birth. Both live in an area (a place like the state of Indiana) that is basketball-mad. Their identical genes make them both taller and quicker than average to the same degree. John goes to school in one city, plays basketball a bit better on the playground, enjoys it more, practices more than most, catches the eye of the grade-school coach, plays on a team, goes on to play in high school where he gets really professional coaching. Joe goes to school in a city a hundred miles away. However, precisely because his genes are identical to Joe’s, precisely because his is taller and quicker than average to exactly the same degree, he is likely to have a very similar life history. After all, this is an area in which no talent for basketball is likely to go unnoticed.
On the other hand, Mark and Allen have identical genes that make them both a bit shorter and stodgier than average. They too are separated and go to different schools. However, they too have similar basketball life histories except in their case, both play very little, develop few skills, and become mainly spectators.
In other words, genetic advantages that may have been quite modest at birth have a huge effect on eventual basketball skills by getting matched with better environments — and genes thereby get credit for the potency of powerful environmental factors, such as more practice, team play, professional coaching. It is not difficult to apply the analogy to IQ. One child is born with a slightly better brain than another. Which of them will tend to like school, be encouraged, start haunting the library, get into top stream classes, attend university? And if that child has a separated identical twin that has much the same academic history, what will account for their similar adult IQs? Not identical genes alone — the ability of those identical genes to co-opt environments of similar quality will be the missing piece of the puzzle.
Note that genes have profited from seizing control of a powerful instrument that multiplies causal potency, namely, feedback loops that operate between performance and its environment. A gene-caused performance advantage causes a more-homework-done environment, the latter magnifies the academic performance advantage, which upgrades the environment further by entry into a top stream, which magnifies the performance advantage once again, which gets access to a good-university environment. Since these feedback loops so much influence the fate of individuals throughout their life-histories, the Dickens/Flynn model calls them “individual multipliers”.
Understanding how genes gain dominance over environment in kinship studies provides the key to how environment emerges with huge potency between generations. There must be persistent environmental factors that bridge the generations; and those factors must seize control of a powerful instrument that multiplies their causal potency.
Were Flynn and Dickens model correct, environmentalism would have been triumphant. As noted before though, this model is contradicted by a number of findings (see: point 6) and is superfluous given the overall negative correlation between the secular increase in IQ and general intelligence, let alone the general lack of strict measurement invariance between cohorts. Flynn and Dickens, nonetheless, made a strong case for it and Flynn’s “What is intelligence” no doubt convinced many. The GWAS findings, though, make a strong version of this model untenable. As the authors of the study note, the findings unquestionably establish a large additive genetic component to (within population) IQ differences.