In March 2016, I wrote the following:

I decided to look at an excellent study that Lynn had cited.  In this study you had 14 pairs of identical twins (one born undernourished, the co-twin born well nourished as measured by birth weight; twin pairs were raised in the same homes).  At an average age of 13, they had their head circumferences measured and were given the WISC IQ test.

The heavier twins had crania that were 0.64 cm bigger than their undernourished co-twin.  At age 13, the within sex standard deviation for head circumference appears to be 1.31 cm, so that’s a difference of 0.49 standard deviations.

When it came to verbal IQ, the well-noursihed twins and the undernourished twins had the exact same average IQ.  And when I saw the exact same average IQ, I mean the exact same average IQ: 98.29 vs 98.29 (unadjusted for old norms)

However when it came to performance IQ, the well nourished twins scored 7.07 IQ points higher than their undernourished co-twins.  That’s a difference of 0.47 standard deviations, virtually identical to the 0.49 standard deviation difference in head circumferences.

So it seems that Richard Lynn was half-right.  Brain size gains caused by  prenatal nutrition do perfectly parallel IQ gains caused by nutrition, but only when it comes to Performance IQ.  Prenatal nutrition seems to have virtually zero impact on Verbal IQ, though given the small sample size (only 14 twin pairs), these conclusions are tentative.

It’s amazing how well this study of identical twins perfectly parallels the difference between North American young adults in the 21st century vs circa WWII.

21st century North American young white adults are to their circa WWII counterparts as well nourished identical twins are to their less nourished co-twin in that their head circumference and Wechsler Performance IQs are both about half an SD larger, while their verbal IQs are about the same.

Because humans are cultural creatures, I believe the brain evolved to prioritize verbal ability during times of malnutrition (which as Lynn noted, includes disease since disease prevents the body from using nutrients), so when sub-optimum nutrition shrinks the brain, mostly spatial IQ suffers, and then when prosperity returns you get a genuine Flynn effect, but it’s 100% concentrated in spatial ability. Spatial ability is a luxury of the well fed.

One might wonder why, with all the increasing education and media, I did not find any verbal Flynn effect on the Wechsler. It’s likely that 21st century education gave us an unfair advantage on verbal tests especially Similarities, but this advantage was negated by the fact that a test created in the 1930s was biased against us (especially in tests of specific knowledge). In other words, two conflicting cultural biases cancelled each other out, thus exposing our true verbal intelligence as unchanged since WWII.