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Notes -
I realise this may come across as stirring the pot, but I hope I've been here long enough to have earned the benefit of the doubt.
In the context of the HBD debate, could someone please ELI5:
The concept of heritability and how it relates or doesn't relate to genetic causes of individual or group differences. I am aware of the "books at home" example. Is that all there is to it?
What precisely g is?
Steelman(!) Turkheimer's position. No, I don't want to hear about his politics.
Roughly summarise the position of Kirkegaard et al.
This whole debate always gets technical so quickly that I very often just get lost. I don't want to rehash the arguments here, I would like to understand the basics. But the waters are often so damn muddied (purposely so, I suspect) that it's very hard to get a grasp of what people are even fighting about.
As a term of art--that is, aside from its colloquial sense--heritability is "the proportion of phenotypic variation (VP) that is due to variation in genetic values (VG)." This is a mathematical concept established on population-level statistics. I am not aware of the "books at home" example or what you take from it, so I'm afraid I am unable to tell you whether that is "all there is to it." But that link to Nature is a pretty short read, if you want to know more about how heritability factors into our understanding of highly heritable traits (like IQ).
G is also a mathematical concept that turns up in statistical analysis. Researchers who assign what are believed to be diverse cognitive tasks nonetheless observe high correlation in the ability to perform well on these tasks. G is the variable assigned to track that correlation. The fact that the correlation exists tends to undermine competing theories e.g. of "multiple intelligence."
Maybe to make it more of an ELI5, it's common in American culture to think of oneself as a "math person" or a "language person," or maybe even more particularly as a "history geek" or a "physics nerd." In all kinds of standardized testing we find it's actually quite unusual to be noticeably bad at, say, linguistic analogies, while being exceptional at, say, calculus. Even things like self-regulation tend to correlate with g--in a study of prison inmates, for example, populations with higher IQs were less prone to violence (PDF warning). While there are cases of extreme divergence (sometimes so extreme we call them "idiot savants"), statistically speaking high apparent cognitive ability is multi-domain.
Note that IQ is not the same as g, but is an attempt to measure g. Note that I also say "apparent" cognitive ability: you might argue that, for example, the ways in which we parse out "separate" cognitive tasks might not actually be separate, or something. It is important, in discussing intelligence science, to recognize just how much of a "black box" our brains still are to us. We can measure inputs and outputs, and we can even get some limited sense of what is happening internally, but beyond that most of our best guesses are inescapably statistical and of limited (but probably not zero) value at the level of individual humans. But in those analyses we find strong correlations of success across task domains (defined to the best of our understanding), which is extremely difficult to explain in the absence of something like g.
I know you've posed this as a "small scale question" but what position?
Again, you're going to have to be more specific.
Thank you! That is exactly the kind of answer I was looking for. Already learned a lot, thanks!
In a lot of HBD discussions I come across people who will claim that heritability is not identical to genetic causation. I do not understand on what basis this claim is made. To quote myself:
So g is essentially a latent contruct that is not directly measurable and IQ is our attempt to come as close as possible?
Turkheimer seems to be the poster child of the position that in-between group differences, especially as related to IQ, are not biological in nature. He very often is cited authoritatively in this regard. I am aware of his political position that this type of inquiry is unsavoury (and I agree to some degree), but I am not aware of the scientific basis of his argument. It seems to be some kind of "everything is heritable, we don't know what might be the case in radically different environments, so who knows?" which just seems nonsensical to me. I am looking for a good steelman of his position but I don't even know what his position actually is.
Likewise, Kirkegaard seems to be Turkheimer's main opponent. I tried making sense of some of his blog posts, but they are all way over my head.
As I said, I am in dire need of ELI5 because I don't even know what questions I should ask to improve my understanding.
E.W. Kirkegaard is nobody's main anything. He's a guy on the internet with a BA in linguistics, who changed his name and moved countries to try to dodge a piddling amount in a libel judgment.
If you're surveying a scientific argument and both parties are addressing internet randos as major players, you're probably in the wrong part of the argument.
((I'll acknowledge here that @DaseindustriesLtd has persuasively argued that credentialism is a form of censorship used to hold HBD down. It's a good argument, but one I find kind of funny coming from HBDers))
Where's a better place to look?
Murray The Bell Curve and Coming Apart. Then on the other side The mismeasure of a man 1996 edition by Gould.
Thank you! What are the main arguments fielded in the mismeasure of man?
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Consider the phenotypical trait "has two arms." Across a population of sufficient n, how "heritable" is "has two arms?" The answer is "not very heritable." Why? Because while there are probably some number of one-armed or no-armed individuals due to genetic mutation, most people who lack two arms have clearly-established environmental reasons for that (e.g. thalidomide, industrial accidents, etc.). So variation in armedness of the population is not predominantly explained by genetics. And yet it is surely true that you have two arms by virtue of your inherited genetic makeup!
On the other side, how "heritable" is "wealth?" The answer depends on your population sample, but often it is "very heritable!" Why? Well, in most places around the world, wealth is inherited (indeed, the word "inherit" originally referred to certain rights, long before we understood genetics) by operation of law. If wealth is concentrated in a few families (which is true in many human populations), then you would actually see a very tight correlation between the variation in genes and the possession of wealth, giving wealth a high heritability coefficient. Now it may well be that "making money" is something downstream of some genetically-affected trait, but still it seems unlikely that creating a clone of Donald Trump would guarantee you a being destined to inherit millions of dollars.
Examples like these have led to some fairly famous arguments that "heritability" ought to be jettisoned. But I find it difficult to take such arguments seriously, as they are in essence asserting that because observed correlations between phenotypical and genotypical variance do not definitively establish causation, we should stop caring about the correlations. This is the kind of attack that can always be raised against statistics--"oh, your correlations are spurious"--but I have yet to see any such attack that did not amount to an isolated demand for rigor. In cases like armedness or wealth, we can be pretty confident in our ability to see the spuriousness of the comparison. With something like intelligence, it's much more challenging, which is why we spend so much time crafting experiments intended to isolate relevant variables.
I don't know what it means to be a "latent construct" in this context. Probably g is not any one thing; cognition appears to be polygenic, so g is not only a function of having the right pieces, but also a function of how well those pieces work in concert. But yeah, IQ is one approach we've taken to, essentially, benchmarking g.
Yeah, I guess the steelman of his position, as I understand it, would be it's very difficult to compare populations, in part because the very process of population selection involves a choice. It's not that "everything is heritable" (armedness isn't!) but more like "a lot of high-heritability stuff is clearly not genetic, a lot of low-heritability stuff is clearly genetically determined, and your methods for selecting populations for analysis already have a variety of biases built in to them, so all you're really doing is laundering those biases through complex math." Whether you buy that argument is probably going to depend a lot on your own priors re: how much intelligence seems genetic, in the same way that armedness seems genetic. It's an oversimplification to be sure, but if you think "Bobby is smart" is more likely the result of the processes that make Bobby wealthy, or the result of the processes that make Bobby have two arms, you will very likely draw different conclusions about whether the heritability of intelligence (which is, undisputedly, quite high!) says something about the genetics of intelligence.
As someone who does not do a lot of statistical analysis, I often find this to be true of these arguments. I am more familiar with the philosophical problems (especially, the problem of induction) and I find even those quite perplexing! So I'm probably of limited help here. But it does seem to me that people like Turkheimer seem to be more interested in muddying the waters than in understanding and explaining observable correlations; this has a way of shifting extremely heavy burdens of proof over to the people who are actually looking at these correlations and trying, however imperfectly, to explain them. It's much easier to doubt an explanation, than to construct one. For me, as someone who is mostly an outsider to these arguments, the fact that any studies continue to suggest a genetic component to intelligence--never mind many studies!--seems significant, given the overwhelming amount of social and political pressure there is for people to adopt blank-slatism with regard to human cognition.
Thank you! That was the key to my confusion. For some reason I thought the comparison that is being made to figure out the degree to which genetic variance is responsible for phenotypical variance is always between siblings (which is of course one of the ways in which you can isolate the effect). I did not think about comparing two randomly chosen individuals.
Unless I am confused again, that's an independent variable you suspect is responsible for change in dependent variables but one which you cannot measure directly.
Could you give an example of what you mean by population selection here? Would Turkheimer argue that the populations we observe differences in are already subject to extremely potent social forces and it is therefore impossible to isolate genetic effects? But what about, e.g., twin adoption studies?
The mainstream opinion among geneticists seem to be that genes define the potential, the environment the degree to which it is reached. I don't think Turkheimer disagrees with that, only with the argument that in-between group differences are genetic.
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