Richard Lynn (circa 1990) believed that if 20th century nutrition had caused average head circumference in children to increase by 1 standard deviation over several decades, then nutrition had also caused IQ to increase by 1 standard deviation over several decades. Thus, nutrition was Lynn’s primary explanation for the Flynn effect.
Then Jensen came along (circa 1998) and argued that since brain-size only correlates 0.4 with IQ (an overestimate), then a 1 standard deviation increase in head size would only cause a 0.4 standard deviation in IQ, and you needed cultural explanations to explain most of the Flynn effect.
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.
The implications for the Flynn effect are unclear because we don’t know how much brain size has increased over the 20th century. If if measured by head size, brain size gains appear to be huge, but if measured by brain weight, they appear more modest.