This article is taken from the May 2021 issue of The Critic. To get the full magazine why not subscribe? Right now we’re offering five issue for just £10.

There are few subjects on which educated people are more likely to be misinformed than the science of human intelligence. Popular myths and feelgood fallacies abound. In his excellent new book, the educational psychologist Russell Warne debunks 35 of them.

Warne was motivated to write the book, he informs us, after discovering that scientific knowledge was sometimes “alarmingly” out of step with public understanding. For example, in a recent survey he carried out with a colleague, 38 per cent of teachers and 47 per cent of non-teachers agreed with the demonstrably false claim that students with higher intelligence test scores “tend to perform just as well in school as the average student”.

Such erroneous beliefs are not limited to those who lack relevant expertise. In another recent study, Warne and his co-authors looked at 29 introductory psychology textbooks, and found that 79 per cent of them contained inaccurate statements about intelligence.

In the Know is divided into seven sections, dealing with, respectively: the nature of intelligence; its measurement; influences upon intelligence; its relevance to education; its relevance to other life outcomes; group differences; and ethical issues. Each of the book’s 35 chapters exposes a different myth. These range from “every child is gifted” to “intelligence research undermines the fight against inequality”.

Intelligence is real, quantifiable and important, and denying those things can lead to tangible harms

Knowledgeable readers will find the book less technical (though considerably more up-to-date) than Arthur Jensen’s The g Factor, but more technical than Stuart Ritchie’s Intelligence: All that Matters or Ian Deary’s Intelligence: A Very Short Introduction.

Overall, the book is interesting, well-written, clearly structured, and meticulously referenced (Warne opts for an in-text citation style, which allows the reader to track down original studies easily). While its main purpose is didactic — to disabuse the reader of certain misconceptions he or she may have — there is a central message running through the book’s 350-or-so pages: intelligence is real, quantifiable and important, and denying those things can lead to tangible harms.

In the first two sections, Warne elucidates what is arguably the most important concept in intelligence research, namely the general factor of intelligence. The g factor, as it is also known, was first identified by the English psychologist Charles Spearman in 1904. He observed that those who performed well in one subject also tended to perform well in other subjects, and he inferred that some general mental ability influences performance on diverse cognitive tasks.

Using a statistical technique called factor analysis, Spearman provided evidence for the existence of this ability, which he dubbed the g factor or g. Strictly speaking then, g is the underlying ability, whereas IQ is the metric for that ability. (So g is like bodyweight, and IQ is like the reading on a scale.)

As Warne points out, g can be distinguished from the various broad abilities that influence performance in specific domains. Spatial ability, for example, is the capacity to understand the relations among objects in space, such as their orientation, movement and rotation. (It is particularly helpful in technical fields like engineering and physics, but also plays a role in sporting performance.)

However, since these broad abilities are positively correlated (persons of above-average spatial ability tend to have above-average verbal ability) their existence does not negate the existence of g. Warne is therefore able to dispel the myth that intelligence is too complex to sum up in a single number.

Today the evidence for g is overwhelming. When the psychologist Wendy Johnson and her colleagues gave subjects batteries of tests, and extracted a g factor from each, those g factors were almost perfectly correlated. In other words, each battery of tests was tapping the same underlying mental ability.

There are several environmental factors with reliable effects on IQ: adoption, education, iodine deficiency and lead poisoning

Warne and his colleague analysed 97 datasets from 31 non-Western countries, and found a g factor could be identified in all but three, which suggests that g is not a peculiarly Western phenomenon. Researchers have even observed a g factor in animals such as dogs, mice and other primates. IQ scores also correlate with anatomical variables like brain size and white matter volume.

In the third section, Warne explains that intelligence is subject to both genetic and environmental influences. For adults living in Western countries, at least 50 per cent of the variation in IQ test scores is attributable to genetic factors. This does not mean, incidentally, that raising IQ is impossible (or even difficult). But it does refute the notion that IQ is just a measure of social class. According to Warne, there are several environmental factors with reliable effects on IQ: adoption, education, iodine deficiency and lead poisoning. Interestingly, however, large-scale interventions have been unable to raise IQ scores drastically. And those that have raised scores often show fadeout, whereby initial gains are lost after the intervention ends.

Warne documents the extensive predictive validity of intelligence in sections four and five. He notes that IQ correlates with many life outcomes, including school grades, income, job performance, health and even mortality (those with higher IQs tend to live a bit longer). Of course, it is not the only psychological trait that predicts success, but its effects do seem to be larger and more consistent when compared to other variables. Warne also dispenses with the myth (popularised by Malcolm Gladwell in his book Outliers) that after a certain point, higher IQ doesn’t make any difference.

In section six, Warne grapples with the vexed subject of group differences in intelligence. He notes that, although men and women have approximately the same average IQ, men’s scores are more variable, meaning that they are overrepresented in both tails of the distribution. When it comes to racial or ethnic differences in IQ, Warne points out that there is no longer any debate about whether such differences exist, but only about their causes.

Some researchers favour a strictly environmentalist account, whereas others believe that both genes and environment are involved. Drawing on five different sources of evidence, Warne places himself in the latter camp, although he suggests it is too early to put a precise estimate on the genetic contribution (beyond stating that it is greater than zero).

Warne considers the political and ethical aspects of intelligence research in the final section. He disputes the claim of some philosophers that controversial theories should be held to higher standards of evidence. And he rejects the argument that intelligence research is somehow “tainted” by its past — is sociology “tainted” because Corrado Gini was a fascist? Warne documents various ways in which the denial of intelligence differences can lead to harm. For example, people with low IQ are more likely to give false confessions. Such individuals often struggle in many areas of life, and cannot simply be expected to “pull themselves up by their bootstraps”.

One minor quibble is that the same material crops up in multiple chapters (though this does allow them  to be read independently). That aside, In the Know is a must-read for anyone who wants to learn more about the fascinating science of human intelligence.