Again I think Shalizi has nicely, if a little convolutedly, explained how the existence of correlations between different cognitive tests inevitably leads to a common factor such as g, and does not tell us anything about whether that factor represents some underlying physical reality.
This sort of thing highlights a general problem with psychological science which is not grounded in more basic physiological science - often the explanations given are simply redescriptions of the phenomenon described - and there is thus no actual explanatory value in the models.
Shalizi also makes some valid complaints about those investigating IQ differences between racial groups and their generally poor methodology looking at just means, variance and correlations, which rather poorly control for covariates with simple regressions, rather than looking at the whole distribution. He also refers to an interesting paper that shows that racial differences are reduced when prior knowledge is controlled for. My concerns with claims to have controlled for all relevant socioeconomic factors and the racial IQ difference persisting stem from objections like this - you'd think a rather more comprehensive attempt would be made to find additional covariates, and rather better attempts at matching for them, rather than naive linear regression, would be used before categorical claims like this were made.
I think I would like to take exception to Shalizi's claims about the modularity of cognitive domains:
"If we must argue about the mind in terms of early-twentieth-century psychometric models, I'd suggest that Thomson's is a lot closer than the factor-analytical ones to what's suggested by the evidence from cognitive psychology, neuropsychology, functional brain imaging, general evolutionary considerations and, yes, evolutionary psychology (which I think well of, when it's done right): that there are lots of mental modules, which are highly specialized in their information-processing, and that almost any meaningful task calls on many of them, their pattern of interaction shifting from task to task....But the major supposed evidence for it is irrelevant, and it accords very badly with what we actually know about the functioning of the brain and the mind."I think the evidence for separate cognitive, as opposed to perceptual, modules is rather over egged.