Stuart Ritchie, “Intelligence: All That Matters,” Kindle Edition, 2015, 160 pp.

This book is recommended by Charles Murray, a credible author I follow. 

There is a strange disconnect between the scientific consensus and the public mind on intelligence testing. Just mention IQ testing in polite company, and you'll sternly be informed that IQ tests don't measure anything "real", and only reflect how good you are at doing IQ tests; that IQ tests  ignore important traits like "emotional intelligence" and "multiple intelligences"; and that those who are interested in IQ testing must be elitists, or maybe something more sinister.

Yet the scientific evidence is clear: IQ tests are extraordinarily useful. IQ scores are related to a huge variety of important life outcomes like educational success, income, and even life expectancy, and biological studies have shown IQ genetically influenced and linked to measures of the brain. Studies of intelligence and IQ are regularly published in the world's top scientific journals.  (But woe to anyone on a college campus that brings up the subject).

This book offers an entertaining introduction to the state of the art in intelligence and IQ, and shows  how we have arrived and what we know from a century's research. It engages head-on with many of the criticisms of IQ testing by describing the latest high-quality scientific research, but is not a simple point-by-point rebuttal: the book makes a positive case for IQ research, focusing on the potential benefits for society that a better understanding of intelligence can bring.

My Notes (as this is on my Kindle I have locations instead of page numbers):

Location 44-54:  Research shows that intelligence test scores are meaningful and useful; that they relate to education, occupation, and even health; they are genetically influenced, and they are linked to aspects of the brain.  The purpose of this book is to give an up-to-date summary of the modern science of intelligence and answer the common questions that are asked about it. For instance… What is intelligence?

Location 54:  Intelligence is a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly, and learn from experience. It is not merely book-learning, a narrow academic skill, or test-taking smarts. Rather, it reflects a broader and deeper capability for comprehending our surroundings, ‘catching on’, ‘making sense’ of things, or ‘figuring out’ what to do.

Location 58:  This definition of intelligence above describes a mental capacity that everyone has to a degree, but what’s crucial for discussion in this book is that not everyone has the same capacity: some people are more intelligent than others.

Location 74-97:  the mid-to-late nineteenth century, which saw compulsory education laws introduced across Europe, was the period where testing intelligence was first seriously considered.  The first true intelligence tests came not from studying eminent individuals with the highest ability levels, but instead those at the bottom of the distribution. In 1904 the psychologist Alfred Binet (1857–1911) was tasked by the French government with creating a tool to identify children with what we’d now call learning disabilities: children who would need special educational attention. Binet wanted something that wasn’t subjective like psychiatrists’ reports, and something that could group children into different levels of disability.

Location 116-143:  During the First World War, the Harvard psychologist Robert Yerkes (1876–1956) designed tests that could be administered to groups of people, rather than individually. These were used to screen new recruits for the US Army, and assess their appropriateness.  When it came to understanding what intelligence actually is, the most important early contributions were made by the British psychologist Charles Spearman (1863-1945).  Spearman’s most famous idea was that there is in fact such a thing as ‘general intelligence’, a property that causes people to perform well on a multitude of different cognitive tests (he called this "g").  However, hanging over all of this early research, from the very start, was the specter of eugenics.

Location 156-165:  After the above introductions, the author covers how the foundations of intelligence research laid by figures like Galton, Binet, and Spearman have been built upon by modern science.

 Chapter 2, This chapter looks through a set of tasks you could expect to complete if you sat an IQ test today. How does a person’s performance on one task relate to performance on the others? To what extent was Spearman correct about ‘general intelligence’?

Chapter 3, shows how IQ test results relate to how people do in school, at work and in life more generally.

Chapter 4 goes ‘under the hood’ to look at the biology of intelligence: how it might have evolved, how it relates to genetics, and what a smarter person’s brain looks like.

Chapter 5 asks whether we might be able to improve intelligence and make people brighter.

Chapter 6, asks why, if there’s all this scientific evidence backing it up, measuring intelligence is still so controversial.

Location 224-309:  Many of the kinds of tests described below were first designed by David Wechsler (1896–1981). It turns out that, to a great extent, intelligence is general. In other words, people who are good at one kind of mental test tend to be good at them all.  The positive manifold of seemingly unrelated tests is one of the well-replicated findings in psychological science.

Location 319-356:  In the 1980s the psychologist Howard Gardner argued that, instead of a general factor of intelligence, there are in fact multiple intelligences. These include ‘linguistic’ intelligence, ‘logical-mathematical’ intelligence, ‘musical’ intelligence, ‘interpersonal’ intelligence and ‘bodily-kinesthetic’ intelligence. Since then, he has added ‘existential’, ‘laser’, and ‘mental searchlight’ intelligence to the list; there are now around ten in total. He argues that people differ on this whole range of intelligences and that the intelligences don’t necessarily correlate together. There’s just one problem with this theory: there’s no evidence for it.  The conclusion was that, in spite of theories like Gardner, regardless of how you test someone’s intelligence, there’s ‘just one g’.

Location 386-400:  One of the most popular models splits general intelligence into two subgroupings, called ‘fluid’ and ‘crystallized’. Fluid intelligence involves working things out on the basis of no previous knowledge. For example, the matrix reasoning test or the reaction time test can be given completely ‘cold’ – people don’t need to use any prior experience to solve the puzzles or to react to the light. On the other hand, ‘crystallized’ intelligence is made up of the sorts of tests (like vocabulary and general knowledge) that rely on information.  But alternative theories exist: for example, one view is that the tests are really grouped together into ‘verbal’, ‘perceptual’, and ‘rotational’ domains.  From around the mid-twenties, fluid intelligence declines and continues to do so right into old age.

Location 410-414:  recently, the psychometrician Han van der Maas proposed a third theory, called ‘dynamic mutualism’ (2006).  Initial evidence has not been kind – one recent paper found a strong g-factor in children as young as two years old.  So much for the “Blank Slate” theory.

Location 426:  The biggest predictor of being smarter in old age is rather obvious, and perhaps somewhat disheartening: being smarter in childhood.

Location 442:  The most important fact is that, despite the sheer diversity of the tests, people doing better at one test usually do better at them all. This ‘positive manifold’ of test correlations gives rise to the general ‘g-factor’ of intelligence.

Location 456:  Smarter people live longer. In study after study, it’s been found that people with higher IQ scores – and, thus, higher general intelligence – tend, on average, to outlive their less intelligent peers. In some studies, IQ score is about as predictive of the risk of death as is smoking.

Location 463-491: Test scores are strongly and unambiguously correlated with two different, though related, measures of education: achievement (that is, your exam results) and duration (that is, how long you stay at school, and whether you go on to college or university). It has been found that, for instance, motivation levels and intelligence both have their own separate predictive effects on school performance (Ritchie and Bates, 2013).

Location 496-567:  The answer to why higher intelligence predicts a later death is that not only will better-educated people be more aware of how to stay healthy, but more qualifications allow entry into better jobs, which bring all the health benefits that more income provides.  Additionally, it’s not too much of a stretch to link higher social class to better health: those in higher classes tend to lead less stressful lives and live in less polluted neighborhoods, with access to better medical facilities. They’re also more likely to take better care of themselves.  Data from a Swedish study of almost a million men discovered that people in the lowest of the nine IQ categories were over three times more likely to die in the 20 years after their testing session than those with the highest IQ scores.  Being 15 IQ points higher in childhood led to a 24-per-cent lower risk of death in the subsequent years

Location 622:  Even after controlling for social class and education, one study found a correlation of r = 0.45 between IQ and social liberalism.  Higher-IQ people are economically as well as socially liberal, tending to be in favor of freer markets (Carl, 2014).

Location 628:  (I found it surprising that) Intelligence differences don’t explain much of the variation in religiosity, but it is a consistent finding, and it’s independent of education level: Yet there does appear to be something about having a higher IQ that makes people less religious.

Location 637:  Only just over 2 percent of people get scores above 130 on a properly ‘normed’ IQ test (the 3 STD dev portion of the bell curve).

Location 714-721:  The evolution of the human brain shows the cranial capacity (skull size) of our ancestors, estimated from the sizes of fossilized skulls, has increased across time indicating that size matters.  But the brute size of the brain is far from the full story on the evolution of intelligence. After all, the brains of elephants are much larger than ours, and whereas they’re smart compared to most other animals, nobody is predicting that they’ll be launching rockets to the moon any time in the near future.

Location 724-728:  As well as the size, the complexity and organization of our brains changed across evolutionary time.  We are still in the dark about the details of cognitive and brain evolution. Exactly how and when our brains became larger and more complex remains to be discovered.

Location 763:  Fifty percent of the differences in intelligence are due to genetic differences. In behavioral genetic terminology, intelligence is 50 percent‘ heritable’.

Location 809:  Outside of cases of abuse or neglect, the things that parents do don’t seem to have a strong effect on their children’s intelligence, in the long run (Harris, 2009). To put this another way, the main reason smart parents tend to have smart children is because of the genes they pass on, not because of their parenting decisions.

Location 834-869:  there is no single gene, or even a handful of genes, for intelligence (though there are some rare exceptions to this rule). Instead, the intelligence-linked genes turn out to be many thousands of tiny needles in the massive haystack of the human genome.  Those with bigger brains have higher IQs. This finding has been highly controversial, but it is now so well established and well-replicated that it’s pointless to deny it.  A recent review of 148 studies (Pietschnig, 2014) concluded that the correlation between brain volume and intelligence test scores is relatively small, at r = 0.24.

Location 917:  Compared to those with lower ability, the brains of higher-IQ people tend to show less, rather than more, activity when completing complicated tasks: this suggests that their brains can more efficiently work through the problems.

Location 920-922:  It’s the frontal and parietal areas of the brain that seem to be most strongly involved in tasks measuring g.  (the urban myth that ‘we use only 10 percent of our brain’ is just that: a myth).

Location 927: Not only have new DNA-based techniques confirmed the validity of the twin study and reinforced our knowledge about the heritability of IQ, but some of the specific genes that contribute to IQ are just beginning to be found. After decades of purely behavioral measurements, MRI techniques allow us to peer inside the skull and begin to understand how the clump of cells within it can produce complex, intelligent behavior.

Location 1056:  The supposed link between breastfeeding and IQ might just be an example of confusing correlation with causation.

Location 1076:  every male Norwegian sits an IQ test as part of their compulsory army service. In 2012 the researchers were able to compare the later IQ scores of those who had been forced to stay in school for extra years with the scores of those who hadn’t (Brinch and Galloway, 2012). They worked out that the extra schooling added 3.7 IQ points per year.

Location 1091-1099:  Pre-school (Headstart in the US) research has found that by the time the children reached adulthood there was hardly any IQ difference between those who got the extra preschool and those who didn’t.  However, even if IQ wasn’t lastingly affected, there did appear to be other extremely valuable effects, such as reduced rates of criminal offending in the preschool group.  It seems that longer schooling, as well as factors like improved nutrition, are beneficial for intelligence.

Location 1102:  Intelligence-test performance has been getting better by around 3 points per decade since IQ testing began, in a phenomenon known as the ‘Flynn Effect’.

Location 1178:  The jury is in. The scientific evidence shows that general intelligence can be measured, that it matters in life, that its variation is related to differences in our brains and in our genes, and that we should be at least optimistic that it can be improved. And yet the idea of intelligence testing is still deeply controversial. Many intelligence researchers shy away completely from even using the word ‘intelligence’,

Location 1220-1224:  The one country that has tested its entire nation’s intelligence is Scotland – it did so in 1932 and again in 1947 as part of the Scottish Mental Surveys.  Every 11-year-old in the country was tested on the same IQ test: both samples included well over 70,000 people.  The data were recently rediscovered and analyzed with reference to sex differences (Johnson et al., 2008). The result, in both samples, was that the average IQ score of girls and boys was exactly the same.  But it’s not quite so simple. Just looking at the average hides two consistent sex differences. The first is that there are differences in more specific abilities: women tend to do better than men on verbal measures, and men tend to outperform women on tests of spatial ability

Location 1245:  The most taboo topic surrounding intelligence, the ‘race-IQ controversy’ flares up every so often and leads to furious debate. One infamous instance was in 1969 when Arthur Jensen wrote a book-length paper contending that educational and intellectual differences between Black and White Americans were partly due to genetics, and that education had failed to equalize them. Another was in 1994 when Richard Herrnstein and Charles Murray published The Bell Curve, a long and detailed book that discussed the importance of intelligence for society, but also argued that different ethnic groups may see different levels of success dependent on their group’s average intelligence. The area is so toxic and scandal-prone that most researchers (and research funders) give it a wide berth. This means that there’s far less high-quality research in this area than in the other topics involving the brain.

Location 1249-1258:  The greatest discomfort occurs when genetics is mentioned.  Yet genes don’t explain 100 percent of IQ differences, and the environment has a substantial role. Also, we know that a host of other factors besides intelligence, like personality, effort, motivation, and a bit of luck, are vital for a successful life.

Location 1281-1290:  As if worries around equality and immutability (unchanging IQ) were not enough, controversy has been stoked by a slew of books and articles that are hostile to the idea of intelligence testing.  The most famous of these opponents was the paleontologist Stephen Jay Gould, who wrote The Mismeasure of Man in 1981. This book, which was updated in 1996, is a  very partial summary of the literature, omitting opposing data that were readily available at the time of writing.

Location 1300-1332: The author of this book is often asked “Why do we need to know about IQ, what’s the point of researching differences in people’s intelligence?’  One of the reason is that finding the factors that might help us preserve our thinking skills into later life is a matter of urgency. This kind of research will also help us understand the diseases of cognitive aging, like Alzheimer’s, that plays havoc with mental abilities and intelligence is one of the foremost parts of what makes us human: knowing exactly what it is, how it’s instantiated in the brain and DNA, and what effects it has on people’s lives is part of understanding the story of our species.