While IQ scores are normed such that the average IQ of British whites or all Americans is 100 at any given time, the tests must constantly be made more difficult to keep the average score at 100, because performance on IQ tests has been rising at a rate of about 3 points per decade (or roughly 2 standard deviations (SD) per century). Meanwhile a similar rise has occurred in cranial capacity, with white male crania being 200 cc bigger in white American men born in 1980 compared to white American men born in 1820. Similarly, white female crania has increased 180 cubic centimeters. I estimate that’s an increase of about 2.2 SD for men and 2 SD for women, so let’s split the difference and say it’s 2.1 standard deviations in 160 years, or 1.31 SD per century. So the biological Flynn effect is 66% as large as the psychometric Flynn effect. Two thirds.
So if IQ performance has been increasing at a rate of 3 points per decade, does that mean that 2 out of every 3 of those point reflects enhanced biology? Assuming the rise in crania measures a rise in real brain size, I think it does mean exactly that. Although Arthur Jensen noted that the correlation between brain size and IQ is only 0.4 so each SD increase in brain size would only cause a 0.4 SD increase in IQ, it seems reasonable to assume that if nutrition has increased brain size by 1.31 SD a century, it has also increased many other properties of the brain that determine intelligence, so I would interpret the rise in brain size as a proxy for a rise in overall neurological power, not brain size only as Jensen implied.
So if real biological intelligence has increased 1.31 SD (20 IQ points) per century since at least the 19th century, why have IQ scores increased 2 SD (30 points) per century? That’s easy, schooling. As scholar Richard Lynn pointed out decades ago, skipping high school causes your IQ to drop by nearly 10 points, but it does not, in my opinion, cause your intelligence to drop 10 points, it simply impairs you on the culturally biased sections of the test, spuriously dragging down your score. Americans today get at least four more years of schooling than Victorian Americans, but correctly for the spurious nearly 10 point IQ boost of attending high school, reduces the Flynn effect from 30 points a century, to 20 points a century (which perfectly parallels the 1.31 SD per century rise in brain size).
However the fact that IQ tests are so sensitive schooling is a huge problem that needs to be solved. For example, 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 writes:
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
This demonstrates that some IQ tests really do come close to the culture fair ideal. People understood that in the 1920s, but decades of post-modern propaganda has brainwashed generations of credulous university students into thinking otherwise. Even the great Arthur Jensen began started getting sucked in by the “no such thing as culture reduced tests” people towards the end of his life, mostly because he was surprised to discover that that the Flynn effect was much larger on the culture reduced tests than the culture fair tests. However what scholar Richard Lynn understood was that non-verbal IQ is more sensetive to prenatal nutrition than verbal IQ is. So in a way both culture loaded and culture reduced IQ tests overestimate how smart people have become, but for different reasons. Culture reduced tests overestimate because they are biased towards spatial IQ which is more sensitive to nutrition and culture loaded tests overestimate because they are biased towards more schooling.
But when one looks at the most culture fair “verbal” test on the Wechsler scales (Digit Span) and the most culture fair non-verbal test (Block Design) on the Wechsler tests, we find that the former had a Flynn effect of 0 points per century and the latter had a Flynn effect of 30 points per century. A statistical composite of both shows a Flynn effect of about 20 points per century (consistent with the both the 1.31 SD rise in brain size), which should be taken as a real increase in biological intelligence.