Between 1958 and 1990, 20% of the children Korea sent abroad for adoption were classified as special needs or disabled, compared to less than 0.1% of those adopted domestically. In 1976, Korea even came up with a plan to phase out international adoption, making an exception for only mixed-race and handicapped children. They eventually walked this back, but it shows how, according to one scholar, "South Korea’s private adoption industry functioned as a replacement for the nation’s social welfare system.” Yet, when scholars eventually studied the outcomes for Koreans adopted abroad, they found that they did cognitively and academically better than natives in Sweden and the Netherlands. It's one of the great mysteries of social science, which I write about here. richardhanania.com/p/were-19…

Since people are talking about the meaning and implications of extremely low IQs, I'll add something I think everyone should understand: an IQ of 70 is used to indicate* mental retardation because of what it tends to be caused by in certain populations. This is a roundabout way of saying that a low IQ indicates mental retardation because of what it means for a person's behavior, which, I'll explain, covaries with its causes. IQs are mostly normally distributed, and IQs represent the influences of multiple different constructs and causes, but they primarily reflect differences in general intelligence. But there's a major deviation from normality at the lower-end of the scale. If you sample well enough, you'll see something like this: The reason is that there are the normal-range causes that produce the normal distribution, and then there are extreme circumstances that produce extraordinarily low IQs. Contrarily, we don't know of anything that produces abnormally high IQs. There's no known one-off mutation that makes someone a genius, but there are many mutations that we now know can make a person extremely unintelligent. Consider this diagram from a recent paper by @AlexTISYoung: A few very rare protein-truncating variants have enormous negative effects on a person's educational attainment! But nothing we know of has enormous positive effects. As the saying goes, it's far easier to break a machine than to throw a wrench in it and make it work. And that makes sense! Your wrench in the gears is likely to break something, not increase efficiency by 50%. If you hit someone in the head hard enough, you can reduce their IQ score, but not in a million years will you turn them into von Neumann. The reason someone's IQ drops after you bludgeon them is more singular and specific than the reasons IQ varies in the general population. A hit to the noggin may leave someone unable to flex their short-term memory, even if their visuospatial rotation capabilities are unaffected. There's some degree of isolation of function and compensation for deficits in the brain. We know this thanks to many observations, like on the effects of neural lesions (I'll link something fun on this topic with the sources at the end). When we say that someone with an IQ of ≤70 is mentally retarded**, we're saying that they lack adaptive behavior—that they're not very bright, and so it makes living life hard. But if a population legitimately has a mean IQ of 70 (and some do!), we'll notice that they're not drooling troglodytes who can't put on their shoes. This is because the reasons for their low IQ are not things that cause specific and extreme deficits, but instead, things that cause normal-range variation, which is far less severe in nature. One of the ways we know IQ is normally distributed is also a way that we know different causes are at work in certain cases of intellectual disability: sibling comparisons! In large Israeli (B) and Swedish (A) datasets, we can see that when a person has a mild intellectual disability, their sibling tends to be less intelligent too. You also see this for heights, autism, or any other highly polygenic trait. But when a sibling has severe—AKA, "idiopathic"—intellectual disability, the distribution of their sibling's IQs is the same as the distribution for the general population. The deficits in adaptive behavior that result from idiopathic intellectual disability are ones that often make life hard to live in discrete and extreme ways. In some cases, you wouldn't even recognize them as being "retarded" because they have conditions like amnesia, where the person is clearly odd and unfortunate, and they score poorly on an IQ test, but they might sound totally coherent otherwise. Normal-range causes lead to linear changes in adaptive behavior; idiopathic mental retardation, leads to a discontinuous decrease in adaptive behavior. Accordingly, an IQ of 70 does not have the same meaning for members of different groups, since if a group with a mean IQ of 70 randomly pulls a person with an IQ of 70, they aren't likely to score like someone who's mentally retarded for idiopathic reasons. Jensen, incidentally, predicted this sort of result based on some observations he made as an educator. "My student said he was looking for a good culture-free or culture-fair test of intelligence and had not been able to find one. All the tests he used, whether they were claimed to be culture-fair or not, were in considerable agreement with respect to children diagnosed as educationally mentally retarded (EMR), by which they were assigned to special small classes offering a different instructional program from that in the regular classes. To qualify for this special treatment, children had to have IQs below 75 as well as lagging far behind their age-mates in scholastic performance. My student, who had examined many of these backward pupils himself, had gained the impression that the tests were quite valid in their assessments of white middle-class children but not of minority lower-class children. "Many of the latter, despite IQs below 75 and markedly poor scholastic performance, did not seem nearly as retarded as the white middle-class children with comparable IQs and scholastic records. Middle-class white children with IQs in the EMR range generally appeared more retarded than the minority children who were in special classes. Using nonverbal rather than verbal tests did not appreciably alter the problem. I confirmed my students observations for myself by observing EMR children in their classes and on the playground and by discussing their characteristics with a number of teachers and school psychologists. My student’s observations proved reliable. "EMR children who were called ‘culturally disadvantaged’, as contrasted with middle-class EMR children, appeared much brighter socially and on the playground, often being quite indistinguishable in every way from children of normal IQ except in their scholastic performance and in their scores on a variety of standard IQ tests. Middle-class white children diagnosed as EMR, on the other hand, though they constituted a much smaller percentage of the EMR classes, usually appeared to be more mentally retarded all round and not just in their performance in scholastic subjects and IQ tests. I asked myself, how could one devise a testing procedure that would reveal this distinction so that it could be brought under closer study and not depend upon casual observations and impressions." The distinction between types of mental retardation that are demarcated by their symptoms and, thus, by their causes, is important to understand. It's why an IQ of 70 for a Japanese person is likely to indicate an extraordinarily severe issue in need of attention, while for a Bushman, they won't have any trouble surviving. A low normal-range IQ score is bad, but depending on its causes, it's likely to come with different symptoms that make it more or less bad. Final note: this does have implications for the use of IQ as a means of mitigating the punishment of serious criminals on death row. Sources: sciencedirect.com/science/ar… sciencedirect.com/science/ar… pnas.org/doi/abs/10.1073/pna… emilkirkegaard.com/p/african… * Indicate, not diagnose. ** This is not how mental retardation is actually diagnosed, despite popular misconception.