The most important thing to understand...

More important than the origin of the universe, more important than climate change, more important than curing cancer, more important than anything else...

Written by James Thompson.

Intelligence is the most important thing of all to understand, more important than the origin of the universe, more important than climate change, more important than curing cancer, more important than anything else. That is because human intelligence is our major adaptive function and only by optimizing it will we be able to save ourselves and other living things from ultimate destruction.

— Douglas Detterman

In these peculiar times, a book about the science of intelligence may sound provocative to those who regard intelligence as a pseudo-science. One hopes they keep an open mind. To those with more knowledge of the science, the challenge is different: how to compress a century of expertise into one volume.

This is a significant book that aims to explain the best evidence on the science of intelligence, simplifying without distorting the results of many complicated publications. Both authors are skilled and knowledgeable. In the 1970s, Richard Haier expanded the field by looking at the functional MRI scans of volunteers as they were solving intelligence test items. These films are marvellous: they show ideas dancing between different parts of the brain like lights switching on and off as tasks and solutions are shunted across the neural network. Roberto Colom has published widely in the field, valiantly testing and often debunking popular ideas about intelligence-boosting training routines and designing computer-based intelligence tests, among many other projects.

The scope of the book is broad, the treatment comprehensive. In 2013, I commended the predecessor volume (written by Earl Hunt alone) as a good compendium, so it is a pleasure to read this one, written in his honour. This is what I wrote then:  

 His approach to intelligence is notably balanced. He concludes that in the big debates the main protagonists have gone well beyond what can safely be concluded from the data. He is quick to pounce on authors who show selective attention to data, quietly suggesting that they are acting as attorneys rather than scientists. He explains key concepts in tutorial fashion, and is willing to spend time clarifying arguments, signalling where the methodological pitfalls lie before jumping in with an opinion, though he does not evade a final judgment. His section on race and intelligence is very balanced, and pays due regard to what we do not know. Some readers will find him too cautious, and no-one can command agreement on their coverage of all topics, but readers will be informed without being misled, and will be wisely guided into a complex and fascinating subject.

Even the history section in this book, which I usually skip, is fascinating. I learned that a good theory of individual intellectual differences had been propounded by Huarte in 1755 (The Examination of Men’s Wits).

Befitting of a textbook, the scope is wide: history, psychometric models, information processing, the brain and intelligence, genetic aspects, the role of experience in the physical and social world, the importance of intelligence in everyday life, population differences, sex differences, the intellectual consequence of aging, the intelligence of nations, and whether intelligence can be boosted.

The authors write clearly, allowing themselves few jokes, but frequent judicious asides, such as explaining to modern audiences what is meant by a man and a woman. They commend individual papers which make essential points, though they usually prefer meta-analyses.

They rightly celebrate the achievements of researchers like James Lee for managing many international groups of researchers and conducting detailed research on the genetics of intelligence, contending that consortia genome-wide association studies are a significant advance in the history of science. 

They rightly point out that although some researchers ascribe great power to the environment, there is still no precise way of measuring the environment and socio-economic status. Indeed, Gary Marks has written about the death of the SES paradigm for Aporia. The bottom line: intelligence remains a better predictor of social class than the other way around.

They also cover recent studies that address critical questions, such as the causal effects of education on intelligence. Small, but real. For cases in which prior intelligence has been tested, each additional year of education was associated with a 1.2-point gain in IQ. For cases in which estimates of intelligence were based on school results, the gain could be even higher (perhaps 5 points). However, as the authors of the textbook note, caution is needed. Scholars do not know if these gains are a one-off with quickly diminishing returns, and they do not know how long the gains persist. And the authors judiciously note that avoiding harms to the brain would be better than searching for costly, time-consuming, and often illusory boosts to intelligence. 

It is a common trope among the uninformed that intelligence does not account for anything much in the real world. Being smart is being book smart, a rather limited ability. But this is not true. The authors show in tables that the relation between IQ and many desirable real-life outcomes is positive and significant. In contrast, the relation between IQ and many undesirable outcomes (e.g., schizophrenia) is negative and significant. Intelligence enhances and protects. 

Other tables show more detailed relations between intelligence and workplace performance—which is high, especially for complex tasks. Despite searching for other contributing factors, including variables related to very specific jobs, a summary of thirty years of research on general and specific cognitive abilities concluded: “Still not much more than g” (Ree and Caretta, 2022).

Like it or not, g is the single best predictor of academic and occupational success. This section should be general knowledge, at least among psychologists.

Hunter and Schmidt had shown that the predictive validity of intelligence on job performance is higher than any other variable, with structured interviews a close second. Recent revisions raise the value of structured interviews, but intelligence remains a powerful predictor of job performance. If organizations ignore this, they will reduce productivity, revenue, and profits by using selection methods of low validity instead. Everybody loses, and the company wastes effort and money. 

This volume is a salutary corrective for those who still contend that IQ is a pseudo-science. Those sceptical readers should start at Chapter 1, which debunks common misunderstandings, and carefully read at least the chapters on the real-world implications of intelligence. The textbook is persuasive because it is prudent and thorough. But of course, it is forced to wrestle with the politically and emotionally provocative material surrounding the topic like a fog. It does a good job. 

The items in boxes are helpful, and the standard of communication is high. The authors want to be understood, and they succeed. Diagrams are also well-used, and the book will sharpen any diligent reader’s ideas. 

Aside from minor issues, are there any major drawbacks to the textbook? There are some things that are relevant to the discussion but are outside the purview of this textbook because they are outside the mainstream peer-reviewed literature. These publications will strengthen the hypothesis that intelligence differences between ancestral groups have a significant genetic origin. For example, admixture studies are a simple but powerful demonstration of the importance of genetic factors. 

Sex and race differences research must meet high standards, but sometimes there was a touch of the Third Reviewer Hurdle in this textbook, in which papers deemed controversial are held to disproportionately high standards. Nevertheless, despite all criticisms, the authors show that international scholastic studies largely support Lynn’s work on the intelligence of nations. As regards sex differences, the authors support the consensus of there being nothing major overall but also lay out the findings supporting Lynn’s claim of a 4 IQ point adult male advantage.

The authors sum up:

Intelligence can be defined; measured; is reliable and valid; tests are not biased against populations when used properly; the general factor is the single most predictive score for a wide variety of real-world outcomes; and is linked to quantifiable brain features that appear to have developmental sequences; individual differences are influenced by genetics although details at the molecular level are only just being investigated; environmental factors are relevant though there is no clear model of their impact on the brain; the sources of average population differences are not agreed but are being investigated; there is no proven way of boosting intelligence though genetics may provide one; being more intelligent is no guarantee of being morally better (though you might be), but enhanced intelligence might help us solve important global problems.

Haier and Colom have written a good book. They look ahead to the improvements all researchers would welcome: a ratio scale of intelligence, which has been elusive, and viable techniques for boosting intelligence. Given the remarkable widespread ignorance on the topic of intelligence, one hopes that this book is widely read.

The Science of Human Intelligence is published by Cambridge University Press.

James Thompson is a former senior lecturer in psychology at University College London. He taught at the University of London medical schools and has a Ph.D. in the cognitive effects of cortical lesions sustained in childhood. His interests are neuropsychology, psycholinguistics, child development, psychological trauma, intelligence and scholastic attainment. Read his Substack, Psychological Comments, here. Follow him on Twitter.

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