The Flynn effect is generally assumed to be 3 points a decade, at least on the Wechsler administered in the U.S.. However my own research in getting a modern sample of young adults in 2008 to 2019 to take the 1937 Wechsler, found they only scored 7 points higher than 1937 norms, suggesting a gain of only 1 point per decade. Of course my sample size was only 17 people so maybe the results will change if I get more data, but then I discovered something interesting.
In the UK, the Raven Standard Progressive Matrices shows a Flynn effect of several points a decade in adults, but only about 1 point a decade in kids (the same as I found for adults on the Wechsler).
Richard Lynn once noted that the Raven Flynn effect is much larger in adults than in kids, a difference he attributed to schooling. Because the generation gaps in schooling are much larger in adults than in kids, schooling contributes to the adult Raven Flynn effect but not the children one, with the latter being a genuine rise in intelligence caused by prenatal nutrition, while the former is mostly spurious.
How does schooling affect a test as culture reduced as the Raven? Lynn argued that it was a disguised a math test that required addition, subtraction and distribution. I don’t buy it. The Flynn effect is supposed to be a fluid test so by definition it shouldn’t require much knowledge. Also, if the adult Raven Flynn effect were driven by learning arithmetic, why didn’t my research find an adult Flynn effect on the Wechsler Arithmetic subtest (in fact I found a negative Flynn effect on that subtest).
Instead I suspect schooling’s impact on the Raven is motivational, not cognitive. Because the Raven is not a fun like the subtests on the original Wechsler, only those who stay in school tend to have the confidence, interest and intellectual discipline to try their best. Those who drop out of school early (specifically the Roma in Serbia) complained that the test was giving them a headache.
Years ago I administered a version of the Raven to a woman in a bar who credited the test with her then passing her exam to attend college (because the Raven made her focus). I also once administered the WAIS-III Matrix Reasoning test (a Raven rip-off added to newer versions of the Wechsler) to a male relative, but he hurried through each item and scored the equivalent of IQ 120. When a female relative scored 135 he demanded to take the test again. This time he agonized over each item, studying the patterns for many minutes, and clocked in at 130.
So Victorian adults would have probably scored around IQ 65 on the Raven, but as kids they probably would have scored around 90. The IQ 90 should be considered a valid measure of their intelligence and makes perfect sense because as Jensen noted, the real component of the Flynn effect is likely caused by the 20th century rise in brain size and predictable from the brain size-IQ correlation. Don’t know the average brain size of Victorians but they were 1.68 SD shorter (11 cm). Assuming their brains were 1.68 SD smaller, and assuming IQ and brain size correlate at least 0.32, we should expect them to have been about 0.32(1.68 SD) = 8 IQ points less intelligent.
Richard Lynn also noted that the Flynn effect being larger on Wechsler Performance IQ than Wechsler verbal IQ is consistent with the nutrition theory because a study of identical twins found that the one born with a smaller head (presumably because of prenatal malnutrition) scored lower on the Wechsler at age 15, but only on the Performance subtests. But this was before the Wechsler added the Raven rip-off on which the malnourished twins would have likely showed some IQ impairment, but not as much as found on hard-core Performance tests. The Raven functions more like a measure of Wechsler full-scale IQ because you can either see the solution (Performance IQ) or talk your way to it logically (Verbal IQ).