Is There a Genetic Component to "Intelligence"?

 
This is not my field and, quite frankly, I don't really care about any differences or similarities in intelligence between different human demes. However, the recent kerfuffle over the intemperate remarks of a Jim Watson have raised a number of interesting issues. One of these is whether there is a genetic component to intelligence.

I have always thought there was. I remember reading Richard Dawkins' take on this subject some years ago. He pointed out that if the average "intelligence" of humans has increased over the past million years by evolution then it follows logically that there were genes (alleles) for differences in intelligence that were selected. It seems unreasonable to imagine that all the alleles have reached fixation so that in today's 7 billion members of the Homo sapiens species there is no genetic variation for "intelligence." (I'm putting "intelligence" in quotation marks because I don't want to get into protracted battles about how to measure it or even how to define it. Let's just agree that there's something called intelligence that exists.)

Up until now I have been under the impression that certain genetic defects resulted in lowered intelligence. For example, Down's Syndrome is strongly correlated with low scores on an IQ test suggesting that the presence of extra chromosome 21 genes affects intelligence. That's a genetic component of intelligence by most reasonable definitions.

In spite of evidence to the contrary, there seem to be a number of people who deny that there's a genetic component to intelligence. One of these is Greg Laden, whose opinion I greatly respect. In a recent posting [Watson’s Lecture Canceled] he even disputes the twin studies that show a strong correlation between intelligence and heritability.

Heritability does not distinguish in and of itself between traits passed on with genes from traits passed on via culture, learning, environment, and so on. For instance, which language a person speaks has a very high heritability value, but this “trait” is entirely, 100% learned with absolutely no genetic component whatsoever. Twin studies have been used to suggest that IQ has a component of heritability that is genetic since it is more correlated in twins than in, say, full sibs. However, non-genetic traits can follow the same pattern. Non-genetic traits can show this pattern because, with respect to environments, full sibs who are not twins do not share the same environment as twins. (And for other reasons.)

It seems to me that Greg is arguing against a genetic component to intelligence. He seems to be going out of his way to discredit any studies that suggest otherwise. I'm not sure where he's coming from on this and that's the reason for posing the question in this posting.

Later on in the comments section of Greg's article he says the following.

We have, on this site anyway, not discussed the fact that neural development in humans does not really allow for much influence from genetics in the way that is asserted by the Rushtonian race argument; we have seen some discussion that the allegations that intelligence = g = IQ = something measurable in a simple way = something that varies across individuals because of allelic differences in some set of genes. But there are a LOT of reasons to not accept this idea, aside from the major disconnect between the genome and the functioning of the brain owing to the actual way real brains actually develop in real life. There has been very little discussion regarding the disconnect between the concept of heritability and the concept of subspecies in animals (race is simply another term for subspecies).

There are so many levels at which this is so wrong that I can’t help but feel … and I’ve said this already … that the Race Concept and the intelligence piece of this are simply not valid scientific arguments, and are almost always either political arguments or arguments being made from ignorance. They are political in their motivation, because the science here is simply operating in a totally different place (a little place I call reality).

Clear as mud. I fully sympathize with the mixture of politics and science that confuses this issue. There are people who want there to be differences in intelligence between races because it fits with their political agenda.

I wonder if there aren't people who believe the opposite because it fits with their political agenda? I wonder if there aren't people who go out of their way to construct pseudo-scientific arguments denying that there can be a genetic component to intelligence. Why would they do this? Because if there's no genetic component to intelligence then there can be no differences between demes and this avoids a messy political debate.

Frankly, I'm not sure if Greg Laden is doing this so I'd like to see some clarification. Is there a genetic component to intelligence? Are there "intelligence" alleles segregating within the human population such that some people are smarter than others because of their parents and not just because of socioeconomic environment? Or, are all differences in intelligence due to environment?

Please, let's not let this thread degenerate into a discussion about racism. Let's talk about the science and stick to the question.

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