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.


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?