Yet another controversial study on the Intelligent Quotient (IQ) has just been published by Nature (30 March 2006), and once again I have a hard time seeing what the controversy is all about. The study, by a group led by Philip Shaw at the National Institute of Mental Health in Bethesda, Maryland, tracked the growth of the brains of 300 children from age 6 to 19. The children were scored for their IQ early on in the study, and divided into three groups according to their IQ score. The researchers noticed that the bottom third of the distribution was also characterized by a thinner cortex (the area of the brain devoted to memory) and especially prefrontal cortex (where reasoning and planning are carried out). Interestingly, however, these children’s cortex eventually caught up with the rest of the group by age 19, recovering normal thickness.
This study indicates that there is some relationship between a measure of cognitive ability (IQ) and some physical characteristic of the brain. While interesting, this should be neither surprising nor controversial: after all, nobody would be shocked by the discovery that someone’s aerobic capacity is a function, among other things, of his lungs’ capacity. Of course mental functioning is related to brain structure: where else would it come from?
Also, the study is entirely agnostic about the nature-nurture controversy: we don’t know if the initial disparity among children was due to genetic or environmental factors (or, more likely, to some complex interaction between the two). Again using the lung-respiration analogy: some people may have larger lungs because of their genetic make-up, though we also know that the environment has a significant effect on shaping lung capacity, because of a widespread phenomenon in the biological world, known as phenotypic plasticity (most of my scientific career has been devoted to that topic, incidentally). Indeed, the fact that the children who started with a low thickness of their cortex caught up by the end of their teenage years seems to indicate a good degree of plasticity in brain growth, and suggests that if we really wish to use IQ for whatever purpose (it is significantly correlated with performance in formal school settings), we ought to measure it repeatedly throughout the growth of an individual and adjust our expectations accordingly.
A long-time critic of IQ studies, Steven Rose (Open University, UK) questioned the study by saying that “performance on cognitive tasks depends on a large number of factors, from emotive state to recall ability, and the IQ approach ignores all of these.” The first part of that statement is certainly true, but then again, it is a bit sterile to argue that one cannot do research because things are just to damn complex. If that attitude were generally adopted by scientists we would have made no progress since the ancient Greeks. The second part of Rose’s criticism misses the point: IQ measures are not meant to pinpoint any particular causal mechanism, they are simply a (probably simplistic) overall measure of a certain type of intellectual performance. Rose is of course countering the equally insane tendency on the other side of the debate, to automatically assume that IQ scores are the result of genetic rather than environmental factors (as in the controversial, and scientifically extremely naïve book “The Bell Curve,” by Murray and Herrnstein). IQ is in fact analogous to lung capacity: it measures something (though the latter is a much more straightforward biologically indicator), but it tells us precisely nothing about the causal pathways producing that particular measure. Therefore, studies like the one by Shaw and collaborators should be welcome as tiny steps toward understanding, rather than either embraced or rejected automatically, depending on one’s a priori ideological positions.