Mental Retardation

People working in the field usually divide cases into organic and familial retardation. Which may not be the best possible categories, but there you are.

A fair fraction of mental retardation is caused by some kind of environmental insult: birth trauma, infections such as rubella, iodine deficiency, fetal alcohol exposure, closed-head injures, etc. I’m not talking about that.

Probably more than half of all cases have some kind of genetic cause, although we have a limited understanding of the genetic details. We know of > 450 genes that can caused retardation when mutated, but there are probably many more. They’re being found at a rapid clip.

If retardation is defined as an IQ below 70, most retarded individuals are only mildly retarded, having IQs between 55 and 70.

People with IQs between 40 and 54 makes up about 10% of the retarded: this is usually called moderate MR. Between 20 and 39, severe (3-4%). Lower still, profound: 1-2%

Mild retardation is more common among ‘minorities’ and low SES people, since those groups have lowmean IQs. A lot more common. The percentage of people in the US with an IQ below 70 must be at least twice as high as it is in Finland or Japan. If you look at young people, more like three times.

As the IQ decreases, the fraction dubbed ‘organic’ increases. At IQ 70, that fraction is higher for whites than blacks.

People with organic retardation often have other physical anomalies (are funny-looking). Generally speaking, their siblings do not show lower-than-average IQ.

The siblings of people with familial retardation show the same sorts of correlations as the siblings of people with +2sigma IQ: they regress up just as much as the others regress down.

I think that genetic kinds of ‘organic’ retardation are generally caused by mutations of major effect, either recessive or de novo. Think of it as a spanner in the works.

Familial retardation looks to be caused (in part) by differences in the number of deleterious mutations of small effect. Sand in the gears. Different mutation rates in different populations would, all else equal, lead to between-population differences in the average number of deleterious mutations of small effect. Differences in average IQ between populations could also be caused by differing selective pressures. Both effects may contribute.

If genetic differences were the main cause of group differences in average IQ, how would you tell what kind of genetic factors had caused this? Well, you could do high accuracy whole-genome sequencing and try to find out the frequency of busted-up genes in the two populations. You could calculate the correlation of IQ with load, within-population. This might be tricky: we can recognize when a coding gene is totally hosed, but it’s harder to evaluate the effect of less dramatic changes. A higher genetic load could be generated by relaxed selection, rather than a higher mutation rate – but a specific mechanism like increased paternal age would cause an increase in a particular class of mutations (mostly point mutations), rather than an across-the-board increase.

If extra load was involved, you would probably  see elevated rates of other kinds of brain malfunction, various flavors of crazy. On the other hand,  weaker selective pressure for intelligence might not imply more insanity: dogs aren’t as smart as humans, which is the fruit of a different selective history, but they don’t seem particularly crazy.

Are there big differences in the frequency of various kinds of lunacy in different populations?  Sure.  Everyone knows that – don’t they?

A single gene variant can contribute to the risk of autism, manic-depression, depression, and schizophrenia, and having a close relative with one of those syndromes ups the risk for all of them. I never thought that DSM categories carved nature at a joint, but they may be worse than useless.

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13 Responses to Mental Retardation

  1. DSM is dismal and sloppy. You are right that it probably impedes proper research. The list of symptoms becomes the holy grail, simply because psychiatry has difficulty agreeing on fundamental causes, so it has to work at a superficial level.
    However, it would be interesting to build up a “sand in the gears” mutation load database to map onto group IQ differences. As to dogs, given that they differ in intelligence, and in aggression, shouldn’t the mutation load explanation be more easily testable with them?

    • engleberg says:

      The multiaxial assessment at the start of DSM IV asks- Are you an annoying retard? Sick enough to be woozy? Teachers tired enough of your jackasssery to hook you on ritalin? No friends? No family? Can’t keep a job? Ever annoy a doctor? Lawyer wants you in the nut hatch to dodge the joint? Want some semi-legal drunk pills?

      OK, not the most sciency science ever, but neither is the Boy Scout Handbook or the Authorized Version. They all have their place.

  2. jb says:

    It used to be, even fairly recently, that large numbers of people died at young ages. Presumably on average the people who died had more sand in the gears (i.e., genetic load) than those who didn’t. Also, until recently ineffective people had fewer surviving children than effective people. But now days almost everybody lives, and there no longer seems to be much correlation between life success and reproductive success, so it would seem that the sand is no longer being swept out of the machinery. I have long had a sense that this ought to be a problem for us (i.e., civilized Western peoples) on some time scale, but I don’t have much idea what that time scale is. Is this something we ought to be worried about now, or 50 generations up the road, or what?

  3. Grantham says:

    “Are there big differences in the frequency of various kinds of lunacy in different populations? Sure.”

  4. William Newman says:

    I myself doubt that DSM carves reality at the joints, but crosstalk between behavioral symptoms of gene-level defects doesn’t seem like very strong supporting evidence. If you mess up relatively low level housekeeping functions in a complicated computer/network system — e.g., cooling failure or power supply instability or disk corruption — it can easily cause different high level symptoms depending on various random factors. It wouldn’t be surprising to me if complicated learning nervous systems had some similar instabilities. If you have a defect in the carburetor or the differential, there are a few characteristic ways that a car goes wrong, but if you have a defect that causes an unreliable network at the factory or unreliable disk storage at the dealership, who knows what will happen to the observed behavior of cars?

  5. syon says:

    MMMM, mean White American IQ is about 100, while the Black American mean is somewhere around 85…..And retardation is set at IQ 70….So, retardation for Whites begins at roughly two SD below the mean while for Blacks it starts at one SD…

    Does anyone know the history behind locating retardation at IQ 70? I imagine that there must have been a bit of maneuvering to find something that was as far as possible below the Black mean, but not so far as to be absurdly low for Whites…

  6. Will says:

    I believe the last sentence is missing a “not”. It should read, “I never thought that DSM categories carved nature at a joint, but they may NOT be worse than useless.” I’m sure all the smart people reading this entry noticed this.

  7. teageegeepea says:

    Is there a good summary of group differences in mental illness online?

    • gcochran9 says:

      I’ll look around fora decent review. Probably more than half of the articles are arguing that psychiatrists are racists, which, since they are on the whole such notorious rednecks, is of course plausible.

  8. As a layman it seems to me that variations within a population in genetically determined IQ imply there are genes which increase IQ but at some cost (like less disease resistance for example). So the overall reproductive effect of such genes is roughly neutral and they can exist at an equilibrium fraction in the population strictly between 0 and 1. So members of the population may by chance get more or less such genes and be smarter or dumber without much affecting their overall fitness.

    The average IQ level between populations can then vary depending on the relative importance of IQ and the competing traits (like disease resistance) in the local environment. So environments where high IQ is more important and things like disease resistance are less important would produce populations with higher average IQ than environments with the reverse.

    Is there some reason this doesn’t make sense?

  9. Matt says:

    This is going to be an obvious comment, but could you do something with looking at the IQs of mental health patients of varying ethnicity?

    Assuming IQ and mental health correlate due to mutational load, if race X are mental health patients more frequently at a given IQ threshold than race Y, then they race X’s difference from race Y should be due to IQ selection rather than load.

    Like if gentiles with schiz hit IQ 85 (hypothetically) while Jews with schiz hit IQ 100, then we can tell the Jews have undergone specific IQ selection, while if they are both schiz at IQ 85 (and presumably this would mean relatively fewer schiz Jews), then the main difference will be more likely due to genetic load (complicating factors would be if selection for increased IQ mainly works through reducing load generally).

    Stronger selection for good mental health and IQ in an environment where mutational load should be higher would make this approach not work.

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