One of the weapons IQ deniers try to use against IQ tests is the fact that culture loaded crystallized tests of intelligence like vocabulary and general knowledge appear to be more heritable than culture reduced “fluid” tests of intelligence that call for spatial reasoning and novel problem solving. This fact has been used by IQ skeptics to argue that so-called genetic innate IQ is really just acquired knowledge and skill, and that there’s no such thing as culture reduced tests or measures of fluid ability.

Admittedly, it is ironic that tests like general knowledge (who wrote the novel Beloved?) and vocabulary (define the word “obsolete”) would be better measures of innate genetic ability than tests that directly measure your ability to solve novel problems, because IQ tests were created in the first place to measure native intelligence independent of school learning, and indeed tests like general knowledge and vocabulary were initially rejected by psychologists as too education loaded to measure innate talent, so why are heritability studies showing the opposite? Scholar Scott Barry Kaufman explains it as follows:

The researchers argue that their findings are best understood in terms of genotype-environment covariance, in which cognitive abilities and knowledge dynamically feed off each other. Those with a proclivity to engage in cognitive complexity will tend to seek out intellectually demanding environments. As they develop higher levels of cognitive ability, they will also tend to achieve relatively higher levels of knowledge. More knowledge will make it more likely that they will eventually end up in more cognitively demanding environments, which will facilitate the development of an even wider range of knowledge and skills. According to Kees-Jan Kan and colleagues, societal demands influence the development and interaction of multiple cognitive abilities and knowledge, thus causing positive correlations among each other, and giving rise to the general intelligence factor.

To be clear: these findings do not mean that differences in intelligence are entirely determined by culture. Numerous researchers have found that the structure of cognitive abilities is strongly influenced by genes (although we haven’t the foggiest idea which genes are reliably important). What these findings do suggest is that there is a much greater role of culture, education, and experience in the development of intelligence than mainstream theories of intelligence have assumed.

I have a different explanation. I think what’s really going on is that verbal ability is more heritable than spatial ability, and since most of the crystallized tests are verbal, and most of the fluid tests are spatial, experts are falsely assuming crystallized scores are more heritable than fluid scores, when really the connection might be largely spurious.

Why would verbal ability be more heritable than spatial ability? Most likely because as social animals who depend on the cumulative knowledge of many generations to survive, the verbal ability to learn from your culture was more useful to most humans than the spatial ability to create tools and shelter on your own, so when the brain suffers numerous environmental insults (prenatal malnutrition, hits to the head, alcohol, drugs etc) it evolved to preserve our verbal abilities and verbal memories over non-verbal intellect. Since verbal ability (which tends to be emphasized on knowledge tests) is less sensitive to environmental damage, it better reflects our genetic potential.

A second reason why knowledge tests might be more heritable than ability tests is that your performance at learning new tasks and solving novel problems vacillates dramatically based on the amount of sleep you’ve had and how much alcohol you’ve consumed or whether or not you have dementia, or simply how motivated you are on a particular day. By contrast, your performance on tests of general knowledge is much less influenced by any of these factors, so rather than measuring your ability and willingness to learn new tasks on a given day (fluid test), crystallized tests reliably measure how much you have learned over an entire life time, allowing factors like fatigue, alcohol, dementia, and motivation, to cancel out. In other words, a reliable indirect measure of intelligence (crystallized knowledge tests) is often more useful than an unreliable direct measure (fluid ability tests).

A third reason why crystallized knowledge tests might be more heritable than fluid ability tests is that the former are just better measures of general intelligence (g) and g is the most genetic component of IQ. A century of research shows that the more complex and abstract a task is, the more it loads on Spearman’s g factor, and acquiring knowledge over a lifetime is a very complex mental task. Acquiring vocabulary in particular requires lots of abstract reasoning because of the very subtle distinctions between how people use words in different contexts, and the many abstract concepts these words describe. By contrast, a lot of fluid tests are spatial, which tends to require less abstraction, but more concrete and mechanical practical hands-on ability.

A final reason why crystallized knowledge tests might be more heritable than fluid ability tests is that we live in a society where almost everyone is so over-educated by endless schooling, television, the internet, etc, that virtually everyone in First World Western countries has the opportunity to learn about virtually everything, so differences in knowledge largely just reflect inequality that is genetic rather than socio-economic. But in studies where there are large differences in social environments, crystallized knowledge becomes less heritable than fluid ability. For example, scholar Richard Lynn wrote:

…improvements in cognitive stimulation would be expected to act most strongly on the verbal-educational abilities which are learned formally and informally in the family and in schools, and least strongly on the visuo-spatial abilities. The verbal IQs of the Wechsler are more highly correlated than the performance IQs with socio-economic status (0.21 and 0.08 respectively in the calculation of Jensen & Reynolds, 1982). Similarly in Dumaret’s (1985) adoption study in France in which lower class infants were adopted by professional families and compared with their half siblings who remained with their biological mothers, the adopted siblings at the age of approx. 9-10 yr showed a 20 IQ point superiority in the verbal scale of the WISC but only an 8 IQ point superiority on the performance scale. The most straightforward interpretation of these results is that the better cognitive stimulation provided by higher SES parents acts more strongly on the verbal abilities. This is further supported by the results of Burks (1928) and Starr and Weinberg (1976) that the adopting parent-adopted child correlation for vocabulary is higher than for other abilities.

In addition, excellent research in the 1920s showed that canal boat children who lived a nomadic existence where they were virtually deprived of schooling, showed massive declines in IQ as they got older. Because IQ tests are normed for age, and because these kids were kept out of school they fell further and further behind their chronological age-mates on the type of knowledge that IQ tests measure. Young canal boat kids would have an IQ around 90, but older canal boat kids would have an IQ of 60. However in a footnote on page 1001 of this document, scholar Arthur Jensen reveals that this extreme social deprivation mostly afflicted crystallized knowledge, while leaving scores on fluid ability preserved:

When the canal boat children were tested on nonverbal performance tests, there was much less decline in scores and the average IQ of the children was 82, which is a typical value for unskilled workers, as the canal boat people were. Fewer than 1 in 10 obtained performance IQs below 70, and in fact there was a slight positive correlation between performance IQ and age.