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.