Dienekes points to a study by Marieke van Leeuwen and colleagues, in which they assess the phenotypic correlation between IQ and brain volume in a sample of 9-year-old children. The correlation overall is between 0.2 and 0.33 for different components of brain volume, consistent with earlier studies of adults based on MRI scanning.
What interested me about the paper was the last sentence of the abstract:
The relation between brain volume and intelligence was entirely explained by a common set of genes inﬂuencing both sets of phenotypes.
which seems quite interesting if true. The paper involved a comparison of the phenotypic correlations in sets identical and fraternal twins, allowing an estimate of the genetic correlation of the traits.
Table 7 shows the genetic and environmental correlations, below and above the diagonal, respectively. Amongst brain measures the genetic as well as the environmental correlations are signiﬁcant, showing that correlations between genetic and environmental factors both contribute to the phenotypic correlations amongst the three brain measures. The same applies for the phenotypic correlations between PO [perceptual organization] and g [the general factor of IQ], and PO and VC [vocabulary and comprehension, another part of the IQ test]; common genetic as well environmental factors contribute to the phenotypic correlations between these intelligence measures.
In other words, when it comes to the correlations within the different components of brain volume, and within different parts of the test battery, environmental correlations and genetic correlations were both important.
In contrast, the phenotypic correlations between brain and intelligence measures and among intelligence measures are explained by correlations between genetic factors only.
The different values for genetic correlations of test and brain volume variables gives them some ability to test one causal hypothesis:
If intelligence causally inﬂuences brain volumes, this would also be reflected in the genetic and environmental correlations: all genetic and environmental factors that inﬂuence intelligence would, through the causal chain, inﬂuence brain volume. However, our study shows that only the genetic correlations are signiﬁcant. In fact 85% to 100% of the covariation between brain volume and intelligence are caused by shared genetic factors. This leaves two options: 1) the relation between brain volume and intelligence is caused by a set of genes which inﬂuences variation in brain volume and this variation in turn leads to variation in intelligence 2) pleiotropy: there is a set of genes which inﬂuence brain volume as well as intelligence.
The discarded hypothesis is, unfortunately, the least credible of the three alternatives anyway, unless you think that higher IQ actually inflates the volumes of people's brains.
The paper could be more clear about the nature of the "common" genetic variants -- what it means is that the genes are held in common between the two phenotypes, not that they have found common genes. That leaves the nature of the actual genetics of the traits completely open (e.g., rare familial variants versus high-frequency variants, regulatory vs. coding, etc.). They do list a number of examples of possible genes (focusing on myelination as a process that might affect both phenotypes), but these are entirely speculative, and not really worth going into at this level.
What is more interesting is the possibility that the genetic correlations mainly arise from early postnatal development:
Possibly, these genetic factors come into play already early in development. Gale, O'Callaghan, Bredow, and Martyn (2006) and Gale, O'Callaghan, Godfrey, Law, and Martyn (2004) showed – measuring head circumference – that brain growth during infancy predicts intelligence in eight- and nine-years-olds, while brain size at birth and brain growth later in life is not associated with intelligence in both these age groups. After infancy children could not compensate for poor brain growth earlier in life. This shows that the relation between brain volume and intelligence already is established between birth and one year of age (van Leeuwen et al. 2008:8).
This is worth further study since the initial postnatal brain growth period and rate have plausibly changed during human evolution. The timing and rate of developmental effects during this time (also very important to linguistic and other cognitive developments) could have been targets of selection in the past.
Also, modularization (or demodularization) of these genetic networks might have influenced pleiotropies between the disadvantages of larger brains (in developmental and energetic terms) and the advantages of learning.
van Leeuwen M, Peper JS, van den Berg SM, Brouwer RM, Hulshoff Pol HE, Kahn RS, Boomsma DI. 2008. A genetic analysis of brain volumes and IQ in children. Intelligence (in press) doi:10.1016/j.intell.2008.10.005