Someone pointed me to a recent study — Ramsden et al (2011) — which seems to have gotten a bit of press (e.g. “Congressman Fattah Cites Study Showing Malleability of IQ in Teenage Brains“). [To summarize the study: 33 kids had their intelligences measured and brains scanned at ages 14 and 18; the test-retest correlation was 0.8; the range in IQ change between intervals was 20 points; and the IQ change correlated modestly with brain differences.]
As for the study, the authors’ comments notwithstanding — ” Neuroimaging allows us to test whether unexpected in measured IQ are related to brain development” — the found IQ changes are in line with what is typically found. Here is a table out of Brody (1992 p. 233; Intelligence, Chapter 8: Continuity and Change in Intelligence). As can be seen there can be considerable change in IQ scores between years, though the volatility both in terms of mean and range decreases with age.
Likewise, as would be expected from the above, the age to age reliability of test scores increases with age.
So the “unexpected longitudinal fluctuations,” per se, are of little interest. What is is that the score changes correlated with brain changes. As such, the fluctuations can not solely be attributed to measurement error and non-ability/developmental differences. Of course, this implies that measures of IQ have more validity than is sometimes thought (i.e. the known fluctuations in scores across development actually track fluctuations in ability, as indexed by cortical volume). Imagine the press if these results were not found (e.g. “Congressman Fattah Cites Study Showing the lack of validity of IQ in Teenage Brains”)!
But what about malleability? The paper clearly shows the changeability of IQ and the brain in this sample, but it’s not clear how much of this runs from the outside in. A while back I pointed to a few studies which found that the causality between IQ and brain volume largely runs from the inside out:
In a more recent paper by van Leeuwen et al. (2009, also see De Moor et al. 2008), the authors argue that if the causal path runs from cognition to brain, then there should be both environmental and genetic correlations between cognition and brain, since there are significant environmental and genetic effects on cognition, which would then be passed onto brain in the causal chain. Since they found only genetic but not environmental correlations between brain volumes and cognition in this study, they argue that only a causal path from brain to cognition or pleiotropy (possibilities 2 and 1 above) are consistent with their data. In the current results, since we also found significant genetic but not environ- mental correlations between brain and cognition, the most likely causal models are either a causal path from brain to cognition or pleiotropy. The same argument could be made from Posthuma et al. (2003)’s study with adults, which also did not find both environmental and genetic correlations between any cognitive and brain volume measures. (Genetic Covariation Between Brain Volumes and IQ, Reading Performance, and Processing Speed)
At least for g. Which is another issue, as the authors of the Ramsden et al (2011) note:
The locations of the grey matter changes associated with VIQ and PIQ changes do not correspond to the anterior frontal and parietal regions associated with general intelligence7 (g factor). It may therefore be the case that g remains relatively constant across ages, but changes in the ability to perform individual subtests depend on changes in sensorimotor skills. It is also notable that although completion of the subtests comprising verbal and performance measures must implicate a network of brain regions, only structural changes in regions associated with sensorimotor skills showed correlations with changes in VIQ and PIQ.
It would have been informative if they extracted g out to see if or to what extent the structural change correlated with it. General intelligence (and genes for it) is largely what gives stability to IQ scores, after all. In an analysis of the Capron and Duyme (1989) study, a study which is frequently cited in support of claims of IQ’s malleability, Jensen found that the adoption effect was not a Jensen effect. One likewise probably would find that the changes here were not g changes.
The take home message of this study (or message worth taking home) is that while cognitive abilities, at least lower level ones, are stable, they are not fixed and can fluctuate, especially during the development period. But there is really nothing new here.
