Arthur Jensen was always a paid up member of the select club of psychologists who actually give intelligence tests, as well as writing about intelligence. He was an educational psychologist who believed that every child could learn, and wanted them to have a cafeteria of learning choices, not an inflexible school set meal. He was fascinated by human intelligence, and bewildered by the vituperation of those who were not, and who shouted down his observations. He was thorough in his work, honest in giving his opinions, and steadfast in letting the results have pride of place. He was also very bright. Like many bright people, he assumed that you were probably bright, which was kind of him.
His first observation was admit that when he tested some children he came out of the assessment session convinced that they were bright. Then he would add up the scores and find that their results were pretty modest. Intrigued by his mistake, he worked out that they had been socially skilled, had presented themselves well, and had fooled him with their charm. Typically, he went on to say that he did not doubt that they would do well in life, because aspects of character other than intelligence can have an influence on success. Equally, he did not cover up his mistake in estimating intelligence, but reported it with interest. He was certainly not a person to boast “I can tell a person’s IQ at a glance”. Early on he noted that presentation during testing was not always an accurate guide to actual mental ability.
Then he took his observations further. He noted that when he tested Black kids of say, IQ 70 they came across as normal and their behaviour in the playground appeared normal. That is, they related pretty well, had the sorts of interests that other children had, and seemed to be street wise. Seemed to be. They weren’t always able to explain the rules and scoring systems of the games they played, so it depended on how closely you examined their understanding. White kids at IQ 70 were often slightly odd. They were sometimes funny looking in their appearance, and more difficult to make a relationship with. They were often somewhat naive.
One explanation for the difference is that intelligence tests do not accurately measure black children’s intelligence. Jensen wrote a book on this topic in 1980 “Bias in Mental Testing” so you can look up the data and his argument in that text. In a nutshell, he found that the tests did not underestimate black intelligence. In fact, they very slightly over-estimated it. He was also doubtful that there were such separate things as black intelligence or white intelligence. In his view there was human intelligence, and the results showed that both people and groups differed in how much they had. (That is something of a simplification, because he also showed there were some differences, not least in the distribution of full scale scores, with black respondents having a more slender distribution, white respondents a more broad and “normal” distribution).
What other explanations are possible, other than test bias? Jensen argued that black children of IQ 70 were normal. It was not an illusion. Assuming a black IQ mean of 85, IQ 70 is but one standard deviation below the black mean. Nothing special, and not specially bad from that population point of view. A full 16% of black kids in the US are below one standard deviation for the black population. They were normal black kids. In fact, in terms of normality alone, though not in terms of ability, they were rather like white kids of IQ 85 who are one standard deviation below the white IQ mean of 100. Nothing special. Normal white kids. A full 16% of white kids are below one standard deviation of the white intelligence mean. Only about 2% of white kids are below two standard deviations, IQ 70, whereas 16% of black kids are (or were, see below).
In summary, IQ 70 is minus 2 sigma for whites but only minus 1 sigma for blacks. Being below IQ 70 is rare for white kids (2%) and pretty common for black kids (16%). Jensen pointed out that there were two routes to mental retardation: 1) simply being at the lower part of the intelligence distribution; and 2) having something wrong with your brain. So, some white kids are retarded because of some injury or illness or genetic disorder, which also makes them “funny looking”, plus some are just naturally dull. A larger proportion of black kids are naturally dull, and some (proportionately fewer) have had an injury or illness or genetic disorder.
Hence, the difference in apparent normality is real, and is explained by a careful understanding of normal variation in each population.
Can we test this explanation? Apart from checking all the facts (which appear to be correct) we have a new development in the last decade or two. It seems that average black IQ in the US is now about 90. If that is so, fewer black children will be 2 standard deviations below the mean. IQ 70 will be 20 points below the mean for them, rather than 15 points below the mean, so probably only 9% of black children will fall below that particular cut-off point.
As a consequence the proportion of funny looking black kids in the under IQ70 range should have gone up a bit in the last two decades, not because black kids are getting genetic disorders, but because there will be somewhat fewer normal looking backward black children. There will still be proportionately more funny looking white kids than black kids below IQ 70, but the different rates will not be so striking as before.
By the way, people with IQ 70 can do lots of things. Humans are spectacularly intelligent even at 2 sigma below the Greenwich Population Mean. A great deal can be achieved, even in a group who, compared to everyone else, are considered to be at high risk.
Finally, the observation that a child can have difficulties either because they are naturally dull or because they have experienced some adventitious insult to their otherwise normal abilities, is not new. Here is a researcher estimating the number of backward children in a population and expressing himself in forthright language:
“We have seen  that there are 400 idiots and imbeciles, to every million of persons living in this country; but that 30 per cent of their number, appear to-be light cases, to whom the name of idiot is inappropriate. There will remain 280 true idiots and imbeciles, to every million of our population.  No doubt a certain proportion of them are idiotic owing to some fortuitous cause, which may interfere with the working of a naturally good brain, much as a bit of dirt may cause a first-rate
chronometer to keep worse time than an ordinary watch. But I presume,
from the usual smallness of head and absence of disease among these persons, that the proportion of accidental idiots cannot be very large”.
Who will be the first to provide the name of the author, the title of the work, and the page number?