We can use the Jim Watson's us-and-them comments to inform ourselves about intelligence and its heritability and malleability. We cannot do better than start with this set of four posts by Cosma Shalizi.
In particular, the last post in the series demolishes g, the general factor of intelligence, whose very definition is based on statistical arguments, and Cosma uses convincing counter-examples that undermine those arguments. (This also means you will need some -- only some! -- knowledge of statistics to follow the arguments).
It's worth reproducing here the concluding paragraphs:
In primitive societies, or so Malinowski taught, myths serve as the legitimating charters of practices and institutions. Just so here: the myth of g legitimates a vast enterprise of intelligence testing and theorizing. There should be no dispute that, when we lack specialized and valid instruments, general IQ tests can be better than nothing. Claims that they are anything more than such stop-gaps — that they are triumphs of psychological science, illuminating the workings of the mind; keys to the fates of individuals and peoples; sources of harsh truths which only a courageous few have the strength to bear; etc., etc., — such claims are at present entirely unjustified, though not, perhaps, unmotivated. They are supported only by the myth, and acceptance of the myth itself rests on what I can only call an astonishing methodological backwardness.
The bottom line is: The sooner we stop paying attention to g, the sooner we can devote our energies to understanding the mind. [With bold emphasis added by me]
In the third post in that series, Cosma also tackles the issue of heritability (which is at the center of claims about race differences), and malleability. One of the examples he uses is interesting: height, which is known to be highly heritable. He then proceeds to show how heritability has very little -- and how environment has so much -- to do with the dramatic growth in height over the last century:
... height is heritable, and estimates for the population of developed countries put the heritability around 0.8. Moreover, tall people tend to be at something of a reproductive advantage. Applying the standard formulas for response to selection, we straightforwardly predict that average height should increase. If we select a population without a lot of immigration or emigration to mess this up, say 20th century Norway, we find that that's true: the average height of Norwegian men increased by about 10 centimeters over the century. But that's much more than selection can account for. Doing things by discrete generations, rather than in continuous time, height grew by 2.5 centimeters per generation. (The conclusion is not substantially altered by going to continuous time.) If the heritability of height is 0.8, for this change to be due entirely to selection, the average Norwegian parent must have been 3 centimeters taller than the average Norwegian. This, needless to say, was not how it happened; the change was almost entirely environmental. The moral is that highly heritable traits with an indubitable genetic basis can be highly responsive to changes in environment (such as nutrition, disease, environmental influences on hormone levels, etc.).
In contrast, the best estimate for heritability of IQ is far lower (at about 0.34) than that of height (about 0.8). And we know about the Flynn effect: a steady increase in average IQ with time -- about 2 to 3 points per decade. Here's Cosma:
The population average IQ rose monotonically, and pretty steadily, over the 20th century in every country for which we can find suitable records, including ones where we can definitely rule out immigration or emigration as significant contributory causes. (If it really is global, and I think we don't know enough yet to say either way, then the idea that it could be due to migration is — peculiar.) The magnitude of the gains are, as these things go, huge: two to three IQ points per decade. As I said in the earlier post, this puts the average 1900 IQ at 70 to 80 in 2000 terms. Let's check how intense natural selection would have to be to explain this. Over a twenty-five-year generation, we're looking at an IQ change of 5 to 7.5 IQ points. Sticking with the usual biometric model, and taking the best estimate of heritability within that model, namely 0.34, we'd have to see a reproductive differential of between 14 and 22 points, i.e., the average parent would have to have an IQ that much higher than the average person. (I am neglecting correcting for assortative mating and for continuous time, which don't change things much.) Since 15 IQ points is one standard deviation, this would imply a huge bias in reproductive rates towards those with higher IQs. Needless to say, nothing of the kind is observed in any of the countries where the Flynn Effect has been documented.
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There is a lot more in those posts by Cosma, so I will have to ask you to spend some time on them. In the following posts, I'll link to some studies whose results are bad news for people who believe that racial and gender differences in IQ have a strong genetic basis.