I didn't mean to go back and forth on this either. However, reasoned argument requires a conversation. I meant only to put a skeptical light on the very strong claim (which I doubt many researchers in the field would back) that:

A) Fluid: also known as G factor. Is determined mostly by genetics. (emphasis added)

The strength of this conclusion is simply not warranted by the published evidence. A strong argument for a substantial role of genetics in intelligence rests on the assumptions below. If any of these assumptions are untrue, the strong argument must be abandoned out of academic honesty. These assumptions are:

1. The G-factor actually measures what the researchers think it measures. The G-factor probably does measure something. However, as suggested by other commenters, there is sufficient evidence to suggest that tests purporting measuring the G-factor could actually be measuring something else--e.g. socioeconomic status.

2. In the twins studies, that the variance in measured intelligence that is observed is sufficiently large to merit a statically significant difference between the test and control groups. In fact, different groups have reported heritabilities for measured intelligence ranging from 0.9 to 0.5 (Nature 388, 417-418). This large range suggests that the data is very noisy and therefore less reliable than the strong argument permits.

3. In the twins study, that the twins have never shared an environment during any period critical to the development of their measured intelligence. This assumption is even more doubtful because the twins (both fraternal and identical) shared the maternal environment of the womb during which time their nervous systems developed. This is the issue addressed in the study from the News and Views article above (Nature 388(6641):468-71). These data suggest that measured intelligence may have a genetic component but this component is actually a far more complex mixture of genes and environment. For example, it seems possible that some genotypes are more or less affected by maternal nutrition. If this were the case, identical twins should be affected more similarly than fraternal twins resulting in an increased heritability.

4. In the twins study, the environment of the separated twins should be substantially different. I understand your point that there are no laws directly discriminating against families with low incomes. However, there is no doubt that adopting a child is expensive--likely one must be at least middle class to be able to afford it. The example of your secretary doesn't really counter my argument because: (i) she is, as you state, middle class; (ii) she works, I presume, at a university, therefore likely lives in a good school district, and likely values education; and (iii) even if you say these aren't true, the evidence is anecdotal and may be represent an exception to the statistics. I am well aware of the expense of adoption as my wife and I have considered this option and have therefore done quite a bit of research on the subject. We live in Germany, and although the procedure is less biased than in the US, it still favors middle class families. Another selection is imposed by child services departments that screen the appropriateness of the adoptive parents, which decreases (but doesn't eliminate) the possibility that they won't be mentally or physically abusive (or otherwise unfit to be parents). These selections err on the side of caution, so families with ANY history of mental illness (depression, etc) could be eliminated. Thus, the selection is strong. A final consideration is that this group is self selecting--this is especially true in cultures where adoption isn't promoted as strongly as it is in the US (which is most other places in the world). Each of these selections works to decrease the variance in the environment in which adopted children are raised, and therefore decreases the apparent effect of environment on measured intelligence.

Although I realize that I haven't shown any of these premises to be patently untrue, I hope that I have cast doubt on the strong argument for a substantial and direct role for genetics in measured intelligence. Indeed, I admit that genetics could play a role in cognitive abilities, but the data doesn't warrant a strong conclusion.

Normally, I would agree that such a "back and forth" would be silly if we were talking about, say, functional efficiency of the kidneys. However, discussions of the genetics of intelligence is a socially charged topic that can have important social and political consequences. Because you read the Gould book, you will know that less than a century ago, many countries and many states passed laws to sterilize people with low intelligence (along with other people of socially undesirable groups). Some of these laws are still on the book. We shouldn't assume that we are so much better that we wouldn't pass equally evil legislation today.

Moreover, as a professor of neurobiology with a degree in cognitive neuroscience (the former of which was invoked in your initial post as a claim to expertise), your claim for a substantial role for genetics in innate intelligence will be taken more seriously. As scientists, I believe we have important responsibilities--especially in the case of socially charged issues such as genetics and intelligence (or aggression, etc). Thus, I think that it is important in a public forum stress the significant on-going debate surrounding these issues.

EDIT: formatting and grammar