Wednesday, September 05, 2007

Underappreciated Evidence Pertaining to the Flynn Effect   posted by Matt McIntosh @ 9/05/2007 02:01:00 PM

To my mind, the most compelling evidence in favor of Flynn Effect gains being real is physiological: it's well known that there have been increases in height concurrent with increases in intelligence in all the countries where the FE has been operative. What's less well known is that there have also been recorded increases in cranial capacity:
Standard metric data from 885 crania were used to document the changes from 1850 to 1975. Data from 19th century crania were primarily from anatomical collections, and 20th century data were available from the forensic anthropology data bank. Canonical correlation was used to obtain a linear function of cranial variables that correlates maximally with year of birth. Canonical correlations of year of birth with the linear function of cranial measurements ranged from 0.55 to 0.71, demonstrating that cranial morphology is strongly dependent on year of birth. During the 125 years under consideration, cranial vaults have become markedly higher, somewhat narrower, with narrower faces.

. . . and in brain size:
7397 post-mortem records have been studied. These comphrhend all 20- to 50-year old men and women who had been autopsied in The London Hospital since 1907. Fresh brain weight, body weight and height were abstracted and analysed statistically according to sex and to year of birth, any person with a cerebral or skeletal abnormality having been excluded. Fresh brain weight in men increased gradually by an average of 0-66 g per year from a mean of 1372 g for those born in 1860 to 1424 g in 1940-a total of 52 g. The weight of the female brain increased by 0-28 g per year from 1242 g to 1265 g over the same period.

Given an increase in brain size and the correlation between IQ and brain size (0.4), it'd be pretty remarkable if there wasn't any corresponding increase in intelligence. Also, in support of Lynn's nutrition hypothesis, there have been correlations found in developed countries between IQ and presence of certain micronutrients:
The relationship between nutritional status and intellectual capacity in 6-year-old children was investigated in 83 subjects of medium-high socio-economic status, without any apparent risk of malnutrition and normal or high intellectual capacity. Nutritional status was evaluated by measuring food consumption, anthropometrical measurements and biochemical indicators (iron status, red cell folate and total plasma homocysteine concentration (tHcy)). IQ was evaluated using the WPPSI test. The relationship between nutritional status and IQ was investigated by multiple linear regression analysis adjusting for socio-demographic variables and sex. There was a significant and positive relationship between iron intake and both total and non-verbal IQ. This was also the case for folate intake and both total and verbal IQ. The fact that these observations were made in children from a developed country, in which their energy and education requirements are met, suggests that their cognitive development may benefit from specific preventive nutritional interventions with these nutrients.

Also, there have been a few studies showing that FE gains tend to be disproportionately located at the left half of the curve rather than the right, which is the nutrition theory would predict given that the less bright people tend to be poorer and thus benefit more than the wealthier (who tend to be smarter) from nutritive improvements.

Finally, from a psychometric angle, there's this paper (though I've only read the abstract) which found that the amount of covariance on test items explained by g has been decreasing as the scores have been increasing. This is what you'd expect if the biological fundamentals underlying g had been improving among the lower end of the range: when you decrease the variance of one component, item covariance attributable to other components necessarily increases.

I think any satisfactory theory of the Flynn Effect has to also take these pieces of evidence into account and unify the whole picture, either explaining them or explaining them away. The only theory on the table that I think does this plausibly is the nutrition-centric hypothesis, though alternative takes are of course welcome.