There are numerous studies that also show that the general intelligence factor is due to physiological effects (e.g. size of different areas of the brain, etc...). How do you explain that?
There is a theory on why when people are trained in a task they do better – it turns out that the amount of “general intelligence” used is less. There are also studies that show this effect with PET scans of the brain:
> Thus, the largest GMR decreases with practice were found on subtests with the highest g loadings. Jensen noted this finding was consistent with what he termed the conservation of g. Namely, with practice and training, tasks become more automatized and require less g.
(from Haier [1])
There is a recent study (2008, te Nijenhuis ,van Vianen , van der Flier [2]) that shows that the increase in RSPM and other scores is because of training and is not an increase in the overall general intelligence [2] (the studies that you cited is all 8 years or older). Incidentally this study cites two of the Flynn papers that you cited (Massive IQ Gains in 14 Nations: What IQ Tests Really Measure, IQ Gains, WISC Subtests and Fluid g: g Theory and the Relevance of Spearman's Hypothesis to Race).
The idea that Flynn proposes (i.e. that general intelligence is increased over time is dismissed). Here is a full quote of the abstract of the paper (if you need the full paper and do not have access to it, msg. me and I will email it for you):
IQ scores provide the best general predictor of success in education, job training, and work. However, there are many ways in which IQ scores can be increased, for instance by means of retesting or participation in learning potential training programs. What is the nature of these score gains? Jensen [Jensen, A.R. (1998a). The g factor: The science of mental ability. London: Praeger] argued that the effects of cognitive interventions on abilities can be explained in terms of Carroll's three-stratum hierarchical factor model. We tested his hypothesis using test–retest data from various Dutch, British, and American IQ test batteries combined into a meta-analysis and learning potential data from South Africa using Raven's Progressive Matrices. The meta-analysis of 64 test–retest studies using IQ batteries (total N = 26,990) yielded a correlation between g loadings and score gains of − 1.00, meaning there is no g saturation in score gains. The learning potential study showed that: (1) the correlation between score gains and the g loadedness of item scores is − .39, (2) the g loadedness of item scores decreases after a mediated intervention training, and (3) low-g participants increased their scores more than high-g participants. So, our results support Jensen's hypothesis. The generalizability of test scores resides predominantly in the g component, while the test-specific ability component and the narrow ability component are virtually non-generalizable. As the score gains are not related to g, the generalizable g component decreases and, as it is not unlikely that the training itself is not g-loaded, it is easy to understand why the score gains did not generalize to scores on other cognitive tests and to g-loaded external criteria.
So what we have for the immutability of general intelligence is a theory, intelligence test results confirming the theory and PET scans of the brain showing this effect.
[1] Richard J. Haier, Ph.D., Positron Emission Tomography Studies of Intelligence: From Psychometrics to Neurobiology
[2] Jan te Nijenhuis , Annelies E.M. van Vianen , Henk van der Flier, Score gains on g-loaded tests: No g
PS: This reply is a little long so sorry for that.