Tuesday, 2 October 2007

Genetics and IQ

Crooked Timber has a thread on a post by Cosma Shalizi that concludes that currently the heritability of IQ cannot be calculated.

I don't want to disagree with Shalizi in general, as I think he's broadly right, but the comments in the Crooked Timber article are enlightening. The question of how heritable IQ is seems to be posed almost entirely in terms of race - as if that is the only way that the question of heritability can be framed.*

Many of the commenters have picked up on Shalizi's claim that the question of IQ heritability is "not a well-posed question". I disagree with this, strictly speaking, given that IQ is ultimately a measure, however poor, of neural function then, at some level of environmental equality, it will be substantially heritable (and thus 'genetic') - this doesn't tell us that current differences in IQ between racial groups are substantially due to genetic differences, but it also doesn't rule out the possibility a priori. Nor does it assume that IQ measures any really extant thing, rather than just being a grab-bag of questions that challenge the brain to a greater or lesser extent.

It is perfectly respectable to think that the evidence for racial (or gender) differences in IQ is poor, as are estimates of heritability, and confounded by environmental factors that are not, and perhaps cannot be controlled for, that IQ tests are not measuring any single unitary thing, and that there is substantial genetic overlap between groups we identify by ethnic labels. You could also note that many of the most vocal proonents of such positions appear to do so for less than savoury reasons.

But so many people seem to need to go beyond this position to claim that there can be no differences between racial groups, that IQ cannot have any heritability under any circumstances, that IQ measures nothing at all, that it has no connection at all with anything we might call 'intelligence', and that there can be no genetically identifiable racial groups. All these latter claims are profoundly false, and it never ceases to amaze me that so many liberal people feel compelled to believe them in order to shut out the spectre of racism. [if you want to see the excesses of the other side of the coin try the people at gene expression].

The Crooked Timber comments also include this gem:
"...the evidence that Down syndrome and similar conditions are linked to unremediable IQ deficits is just about as weak as the race-linked evidence, for many of the same reasons; see my article in Disability and Society, here – http://home.vicnet.net.au/~borth/DOWN1.HTM"

* I think this is because people are unable to understand that high heritability estimates for IQ do not mean that racial differences in IQ must be due to genetic differences. My favourite example of this is schizophrenia. We know that schizophrenia is heritable, with much greater chance of developing it if close family members have it, and monozygtic twin concordance of around 50%. There have also been a number of genes identified that confer a risk for developing schizophrenia. Now black people in the UK and the US (African Caribbean/African American ethnicity) have much higher rates of schizophrenia than the rest of the population. And this doesn't seem to be due to racist or culturally insensitive doctors overdiagnosing it (doctors from the Caribbean confirm that these people really do have schizophrenia). So there you go, must be due to black people being genetically predisposed to schizophrenia? Nope, because black people in Africa and the Caribbean don't have this elevated rate of schizophrenia. We don't know quite what causes this elevated rate, it doesn't seem to be due to poverty or migration, and many people suspect it is due to the neuropsychological impact of racism or being in a ethnic minority within a wider society.


Anonymous said...

Crooked Timber erroneously presumes genes and environment are totally separate variables. In fact, these variables synergize. Lower IQ folks live in less desirable environments because they are too dumb to care, or to improve their condition (e.g., Sub-sahaarn Africans would still be living in mud huts if colonists hadn't shown them better housing technique)

pj said...

But Cosma Shalizi doesn't. Which is the whole point of his discussion about gene-environment interactions.

If the environment of sub-Saharan Africa is so bad, why are there so many white ex-colonials there?

Your subtext that low IQ groups are somehow doomed to crap environments seems rather contradicted by the evidence that IQ has clearly risen precipitously in the West, and indeed is still rising as the Flynn effect shows. How could this be possible given that Western peoples started out with very low IQs by modern standards?

potentilla said...

Did you link that particular GNXP comment thread because of specific comments, or because it was a handy recent one? and by "people at GNXP", do you mean the bloggers or the commenters? It doesn't seem to me that the bloggers are prone to "excesses", but I would be interested to know if you do, and why.

I wish someone would write a comparison between Cosma Shalizi's post and various ones on GNXP over the last few years which look just as evidence-driven and well-referenced but come to different conclusions. There is too much data and the required level of statistics is too high for me to be able to take a sensible view by myself.

I agree with your main point; that I don't see why discussions of the heritability of IQ or intelligence immediately turn into political slanging matches about race and (to a lesser extent) gender. It seems to me not implausible that there are small average population differences on all sorts of genetic propensities both between "races" (however defined) and the the two sexes; but maybe I am more open to this possibility because I understand that any such differences say nothing whatsoever about individuals and not a lot about social policy.

pj said...

I referenced that GNXP thread because it was a set of comments on the Shalizi post - rather than because it was particularly extreme in and of itself.

I was mainly referring to the commenters, some of whom, even in that thread, can rant and rave about how the stupid are outbreeding their genetic superiors in some dysgenic cataclysm. Or speculate on the percentage white ancestry in African Americans to explain why they aren't as stupid as black Africans, or how only clever foreigners emigrate to the US to explain why various racial groups do better in the US environment than at home.

I'm open to the possibility of genetically driven differences between races and genders - but I'm far from convinced that we have any good evidence that such differences exist, particularly when you look closely at the quality of the data.

But I think at root you are right, it is fear of the consequences that drives most of the Crooked Timber commenters to reject the possibility a priori. You get a similar phenomenon with genetic determinism, the selfish gene, and evolutionary psychology.

Anonymous said...

Here is a critique which also mentions Shalizi's comment, and why he is missing the point:

"Jake, this is a good review and I agree with many of your major conclusions. However, your summary of the literature on g has several problems.

[g-factor] s predicated on the notion that performance across different cognitive batteries tends to be positively correlated

A quibble -- the positive correlation between performance on different test items is not just a notion but an empirical observation that has been supported by millions of data points over the last century. More on this below.

Psychological tests for g-factor use principal component analysis -- a way of identifying different factors in data sets that involve mixtures of effects.

Factor analysis, not PCA, is the method used by psychometricians. They are similar in principle but not in application.

g-factor is very controversial.

Not among intelligence researchers.

In this review, we emphasize intelligence in the sense of reasoning and novel problem-solving ability (BOX 1). Also called FLUID INTELLIGENCE(Gf), it is related to analytical intelligence1. Intelligence in this sense is not at all controversial...

[These authors go on to explain that in their view Gf and g are one and the same.]

From another review:

Here (as in later sections) much of our discussion is devoted to the dominant psychometric approach, which has not only inspired the most research and attracted the most attention (up to this time) but is by far the most widely used in practical settings.

This was published over a decade ago. The psychometric approach has continued to attract the most research and attention and is still by far the most widely used.

The second and broader critique of this work is whether the tests that we have for "intelligence" measures something useful in the brain.

There's wide agreement that the tests measure something useful about human behavior:

In summary, intelligence test scores predict a wide range of social outcomes with varying degrees of success. Correlations are highest for school achievement, where they account for about a quarter of the variance. They are somewhat lower for job performance, and very low for negatively valued outcomes such as criminality. In general, intelligence tests measure only some of the many personal characteristics that are relevant to life in contemporary America. Those characteristics are never the only influence on outcomes, though in the case of school performance they may well be the strongest.

A more standard criticism of g:

while the g-based factor hierarchy is the most widely accepted current view of the structure of abilities, some theorists regard it as misleading (Ceci, 1990).
that is:

One view is that the general factor (g) is largely responsible for better performance on various measures40,85.A contrary view accepts the empirical,factor-analytic result, but interprets it as reflecting multiple abilities each with corresponding mechanisms141. In principle, factor analysis cannot distinguish between these two theories, whereas biological methods potentially could10,22,36. Other perspectives recognize the voluminous evidence for positive correlations between tasks and subfactors, but hold that practical, creative142 and social or emotion-related73 abilities are also essential ingredients in successful adaptation that are not assessed in typical intelligence tests. Further, estimates of individual competence, as inferred from test performance, can be influenced by remarkably subtle situational factors, the power and pervasiveness of which are typically underestimated2,136,137,143.

The concepts of IQ and g-factor have been questioned by several authors. Stephen Jay Gould actually wrote a whole book -- The Mismeasure of Man -- trying to debunk the assumption that intelligence can be measured in a single number. (For a more recent and excellent critique, I recommend this article by Cosma Shalizi.) The common theme among many of these critiques is that the tests for intelligence conflate numerous separable brain processes into a single number. As a consequence, 1) you aren't sure what you are measuring, 2) you can't associate what you are measuring with a particular region (the output may be the result of an emergent process of several regions), and 3) you may be eliding significant differences in performance across individuals that you would recognize with a better test.

You give too much credit to Gould and Shalizi. Their primary criticisms are entirely less reasonable than the points you make.

The main thrusts of their arguments are that test data do not statistically support a g-factor. Gould's argument is statistically incompetent (for a statistican's critique see Measuring intelligence: facts and fallacies by David J. Bartholomew, 2004). Shalizi's criticism is incredibly sophisticated, but likewise incorrect. In a nutshell, Shalizi is trying to argue around the positive correlations between test batteries. If those correlations didn't exist, his argument would be meaningful. However, as I noted above, these intercorrelations are one of the best documented patterns in the social sciences.

significant differences in performance across individuals that you would recognize with a better test.

It's possibly not well known that enormous efforts have gone into trying to make tests that have practical validity for life outcomes yet do not mostly measure g. See for example the works of Gardner and Sternberg. The current consensus is that their efforts have failed. A notable exception might be measures of personality.


Ultimately, we need to use biological measures such as cortical volume to determine what g really is. One possible approach is to combine chronometric measurements (e.g. reaction time) with brain imaging studies. Genetically informed study designs have a role to play here too.

[1] www.loni.ucla.edu/~thompson/PDF/nrn0604-GrayThompson.pdf
[2] www.gifted.uconn.edu/siegle/research/Correlation/Intelligence.pdf"