I found some novel data to test Richard Lynn’s theory that the Flynn effect is caused by better nutrition improving the size and functioning of the brain. CDC head circumference data from circa 1978 and circa 1991. Although head circumference is only a rough proxy for brain size in adults, it’s an excellent proxy in young children, correlating as much as 0.93.
Circa 1978 data (sorry it’s hard to read):
Circa 1991 (data)
I ignored children under age 1 because the age categories for these were not comparable in both eras. But for ages one through seven I found the following increases in U.S. head circumference from circa 1978 to circa 1991 (expressed in 1978 standard deviation units).
1 year: +0.12 SD (males) +0.29 SD (females)
2 years: +0.24 SD (males) +0.28 SD (females)
3 years: +0.21 SD (males) +0.47 SD (females)
4 years: +0.19 SD (males) +0.06 SD (females)
5 years: +0.29 SD (males) +0.53 SD (females)
6 years: +0.22 SD (males) +0.17 SD (females)
7 years: +0.35 SD (males) +0.3 SD (females)
Amazingly, there was an increase for all 14 age sex categories. You would think just by sampling error I’d get at least one decrease or at least no change, but no. This proves the head circumference increase is a real phenomenon.
Averaging across both sexes and all age groups, the increase was 0.27 SD from circa 1978 to circa 1991 or 0.21 SD a decade. Performance on childhood U.S. IQ tests like the WISC has also increasing by 0.2 SD (3 IQ points) per decade since the test was first published in 1949.
Coincidence?
It’s strange to think that in a country as rich as America, the average person was malnourished to the point of brain damage as recently as the 1970s, but when I watch old clips from Donahue or some my favorite 1970s slasher films, I can kind of see it.
The interesting question is whether the entire Flynn effect is a biological gain in real intelligence (as Richard Lynn believed it to be, with a few exceptions) or whether it’s only about 40% biological as Arthur Jensen believed (citing a roughly 0.4 correlation between IQ and brain size) with the remaining 60% being test sophistication caused by culture. Or as James Flynn himself has argued, is the distinction between real intelligence and culturally acquired skills a false dichotomy?
Pumpkin Person 
Increase in circumference definitely would be an increase in cortex surface area. And more complex wiring.
I used to be mildly curious about possible links between macrocephaly and autism savant syndrome. I stopped following the topic around 1999 but apparently the head circumference growth charts have been scrutinized a lot since then. You might have already looked at this, or I might just be opening up a can of worms, so I’m inclined to apologize for that, but anyway I just looked at this 2013 article and it has some bibliography, the last of which I looked up the exact link for.
RETHINKING HEAD SIZE IN AUTISM: SCIENTISTS QUESTION STATISTICS ON “EARLY BRAIN OVERGROWTH” https://iancommunity.org/cs/simons_simplex_community/head_size_in_autism
Compared to what? Early brain overgrowth in autism and the perils of population norms. https://www.ncbi.nlm.nih.gov/pubmed/23706681
Honestly I’m not sure what these might imply about inferring a Flynn Effect from head circumference growth trajectories, but I thought it might be useful for you for me to draw your attention to them in case you hadn’t already known about them.
I can barely read the charts you posted but the ‘number of cases examined’ seems in the range I recall — small enough for there to be possible sampling group error, say from one hospital district in an economically depressed part of an economically depressed part of the country, for example. In the abstract at the the last link I gave it mentions an N of 75,000, so later numbers establishing later charts might be better, so there may be instances where it’s better to compare two recent sets of measurements over a shorter time period than two from long ago separated by a longer time interval. In any case, head size changes over time sounds like a topic with potential, good luck! (One more thing- the growth over time in the rate of cases of autism diagnosis seems to have some serious confounds, but it’s interesting to imagine the possibility that the risk of ‘brain overgrowth’ macrocephaly autisms increases with larger head sizes happening in human species-typical genomes designed for development in conditions highly different from modern environments. Kind of like the way almost everyone who’s lived to be 110 years old or more has been around five feet tall.) (Apologies if I’m rambling – insomnia, middle of the night, etc.)
Yes, I can see how scientists using reference norms that are a decade or two too old could result in many autistics being wrongly diagnosed as macrocephalic.
Growing up I remember it seemed like everyone and their brother was classified by our school board as gifted (top 2% in IQ). In the East Asian family that lived across the street, all three of the boys were gifted! The gifted class was overflowing with students and then just when you had thought you had met them all, there was the gifted guy who is so afraid of being labeled a nerd that he overcompensated by skipping gifted class for a month and goes around acting hyper-cool (yo whad up dude?)
I knew it was a smart town, but how could so many people be gifted, and so many from the same families? When I learned about the flynn effect, I realized the IQ test our school board had been using the same IQ test since the 1970s, so by the 1990s, it was inflating everyone’s IQ by 6 points, which makes a huge difference in the percentage diagnosed as gifted
And now these same people phone me up and say “I’m gifted why didn’t I amount to anything?” Because you were never gifted in the first place, I try to explain. I call it the Mug of Pee syndrome.
A more serious case of bad reference norms distorting science is SIDS. For years it was thought that kids who died from SIDS had enlarged thymus glands that were putting pressure on the trachea, suffocating the infant during sleep. A whole generation of kids had thymus procedures to avoid SIDs. It turned out that SIDS victims did not have enlarged thymus glands at all, they just seemed that way because the reference norms were based on poor kids (the most accessible corpses) and these have shrunken thymus glands.
Having said all that, I do think huge heads are more common in autism, just based on personal observation. Autism appears to be a disorder that disproportionately strikes upper class high (math) IQ families, and building such a large complex brain is a very recent evolutionary development so evolution has yet to weed out all the things that can go wrong. Perhaps kids inherit DNA for hyper-brain growth from their high IQ parents, but something goes wrong in the developmental timing and they end up with autism or other issues instead.
That table isn’t hard to read at all.