Summary of Intelligence: All That Matters by Stuart Ritchie

  • Post category:Summaries
  • Post last modified:January 24, 2024

Summary: 8 min

Book reading time: 2h

Score: 8/10

Book published in: 2015

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  • Intelligence is the name given to the ability to think, learn, and complete mental and intellectual tasks.
  • Intelligence is not fairly distributed. Some have more than others.
  • Women are slightly more represented in the average intelligence groups while men are more represented in the low and high intelligence groups.
  • Intelligence tests were created to spot the low-IQ kids and help them out. They were later used to find the high-IQ kids.
  • The g-factor is someone’s intellectual power.
  • Intelligence is split into two types:
    • Fluid: working things out without previous knowledge.
    • Crystallized: working things out based on previous knowledge.
  • Intelligence correlates with a variety of things such as education, social class, face symmetry, grip, and more.
  • Genes explain 80% of the variation in intelligence.
  • Parenting style has no effect on intelligence.
  • High IQ is likely caused by better connectivity in the brain between white and grey matter.
  • It’s not possible to increase IQ.

Table of Contents

What Intelligence: All That Matters Talks About

Intelligence: All That Matters is a book written by the intelligence researcher Stuart Ritchie. It summarizes the findings about the study of intelligence from its beginnings in the 1960s until today.

It’s a great book to understand what IQ, g, and intelligence are and what we know about them.

It didn’t delve too much into the controversies either.


Get the book here.

Summary of Intelligence: All That Matters Written by Stuart Ritchie

Chapter 1: Introducing Intelligence

Smart people don’t like the idea of intelligence.

Despite its mainstream repudiation, IQ is a significant measure with real-life applications.

But what is intelligence?

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.


Everyone has intelligence, but some have more of it than others. IQ research is the study of these differences.

We’ve been writing about intelligence since at least the Ancient Greeks, but the development of intelligence measurements only came about in the mid-19th century.

Arthur Jensen speculated that the lack of universal schooling (where intelligence differences are the most striking) coupled with the belief that rationality was gifted from the gods didn’t compel people to study intelligence.

Indeed, measures of intelligence were developed when public education began, firstly by Francis Galton (Charles Darwin’s cousin) in the 1860s. Galton wanted to understand the origin of differences between the “eminent families” and the “normal” ones.

So he weighed up the effects of socialization and heredity, and coined the phrase “nature and nurture”.

He collected data from over 9300 people to see if he could detect a pattern, but never found anything significant.

In 1904, the French psychologist Alfred Binet was tasked by the government to design a test to measure and identify children with “learning disabilities”.

Once the test was designed, other psychologists applied it to find the smarter children.

During the First World War, Robert Yerkes (1876–1956) designed group tests used to screen the new recruits for the US Army and assess their appropriateness for officer-level positions.

Finally, the idea of one general intelligence was conceptualized by the British Charles Spearman (1863-1945).

In the early 20th century, Thomson and Burt (UK) worried that too many high-IQ children weren’t using their capacities optimally.

Their work led to the creation of the 1944 Butler Education Act establishing grammar schools, where children who got a high enough IQ score at age 11 would go on to learn a complex curriculum including Classics and mathematics.

This system no longer exists because of its association with eugenics.

The selective breeding of healthier, taller, smarter humans, while discouraging the reproduction of those who weren’t any of these things, was pushed by many progressives in the late 19th-early 20th century.

This led to many eugenics practices in the US, Canada, and the Scandinavian countries for decades.

While eugenics is a political idea, intelligence is a hard, cold, scientific fact.

It’s important not to mistake one for the other.

Chapter 2: Testing Intelligence

A real intelligence test takes over one hour to complete – beware of the ones you do online that last two minutes.

IQ tests you can take:

  1. Wechsler Adult Intelligence Scale (WAIS)
  2. Kaufman Brief Intelligence Test (K-BIT)
  3. Woodcock–Johnson Tests of Cognitive Abilities (WJ)
  4. Raven’s Progressive Matrices
  5. Cattell Culture Fair Intelligence Test (CFIT)

IQ tests measure reasoning, memory, knowledge, mental processing speed, and spatial abilities. The people who are good at one of them are usually good at the others -> intelligence is general (this is the positive manifold).

Population IQ scores plotted on a graph look like a normal distribution.

image 2

In the 1980s psychologist Howard Gardner argued that there are multiple types of intelligence and not just one. But there is no proof for it.

The abilities measured by the IQ test have one thing in common: the g-factor, someone’s intellectual power or energy (the g-factor explains around half of people’s overall intellectual differences).

The g-factor is everywhere but it doesn’t explain everything about mental abilities. Each particular test requires some specific skills which Spearman called the s-factor.

Some subtests measure more g and some, s. The extent to which a subtest measures g is called g-loading.

Tests with strong g-loadings involve complex thought, like matrix reasoning. Tests demanding reaction time involve less g.

Some modern psychologists have theorized that processing speed might be the key to this mental energy, but there’s no proof of that.

Others have proposed that working memory may be the mechanism that underlies g.

In any way, g is only a part of intelligence.

One of the most popular models splits general intelligence into two subgroupings:

  1. Fluid: working things out without previous knowledge.
  2. Crystallized: working things out based on previous knowledge.

Crystallized intelligence keeps on developing as we age while fluid intelligence declines from the mid-twenties. Smarter people experience less decline as they age than those with lower IQs.

Chapter 3: Why Intelligence Matters

Smarter people live longer. In some studies, IQ scores are as predictive of longevity as smoking. It’s believed that this is because smarter people tend to live healthier and exercise more.

Smarter people are also less likely to suffer from psychiatric conditions.

Let’s see what else IQ scores correlate with.

  1. Education: high-IQ people achieve better results at school and study for longer.
  2. Workplace performance: high-IQ people perform better at their jobs.
  3. Income: smarter people tend to earn more.
  4. Social class: smarter people tend to get into higher social classes.
  5. Face symmetry: high-IQ people tend to have a more symmetrical face.
  6. Politics: high-IQ are more liberal (left-leaning) and more interested in politics.
  7. Religion: there’s a weak (r=0.25) negative correlation between IQ and religious beliefs.
  8. Grip: intelligent people have a stronger grip.
  9. Committing fewer crimes: Intelligent people commit fewer crimes.
  10. Being taller: intelligent people are taller.
  11. Liking’ The Godfather on Facebook
  12. Making funnier jokes
  13. Brain size: high-IQ people have bigger brains.

We’re not sure if IQ causes all of these. Some believe that IQ simply correlates because a high IQ is an indicator of good health and development. Others believe that genes that give healthier bodies also give healthier brains.

Only 2% of people have a 130 IQ. Are these people really more skilled than those at 115 and 120?


High IQ scores are related to particularly impressive achievements.

There seem to be no limits to IQ. The higher it is, the better the outcomes.

Despite what we hear in the mainstream narrative, there’s no downside to high IQs. Smarter people aren’t in any way “less social”.

The only negative correlation is short-sightedness. High-IQs are likelier to need glasses. And some studies show that they’re more likely to be bipolar.

To a certain degree, life is an IQ test.

As our societies move towards even more complicated technology and computerization in everyday life, the importance of intelligence – the ability, after all, to catch on to new ideas and solve new problems – will only increase.

Chapter 4: The Biology of Intelligence

Why are humans intelligent?

The evolutionary explanation is that we evolved this way to fight off predators.

If we look at the evolution of the brain, it has gotten bigger over time. Big brains make up for a more intelligent life.

Now, why are some people smarter than others?

The first reason is genetics.

Twin studies have shown that identical twins have more similar IQs than fraternal twins. Further studies estimated that 50% of IQ is heritable (this means that genes explain 50% of the variation). But this effect is stronger in adults (where 80% of IQ is heritable) than it is in infanthood (20%).

Attention: this figure doesn’t concern individuals, but groups.

This means that the role of biology is increasingly important as we age.

What about the environment?

First, the shared environment (which is the things twins go through together, like parents’ education) has little effect on IQ.

The main reason smart parents tend to have smart children is because of the genes they pass on, not because of their parenting decisions.

As of now, we haven’t been able to identify which genes (or groups of genes) are responsible for higher intelligence. We also know that intelligence is polygenic, which means that g is the result of many genes with each having a tiny effect.

We also know that older parents pass on more minor genetic errors to their kids which increases the risk of lower IQs.

Once we know which genes are responsible for intelligence, we’ll be able to test for them and parents will be able to select high-IQ embryos. Of course, this will cause ethical dilemmas.

High IQ is likely caused by better connectivity in the brain between white and grey matter.

The efficiency with which messages can pass between the frontal and parietal brain areas might be the foundation of intelligence differences.

Chapter 5: The Easy Way to Raise Your IQ

It’s not possible. IQ is fixed. However, raising the general IQ of a population would translate to considerable gains in efficiency.

Every way that has been tested to raise IQ (listening to Mozart, brain training, etc) didn’t work.

Now, it’s true that improving your health can improve your IQ (removing worms, removing lead from gasoline, eating a proper diet), but this effect is likely caused by the amount of life force the body can redirect to the brain after fighting off these adverse elements.

Regarding the breastfeeding-IQ correlation, it’s just that high-IQ mothers are more likely to breastfeed – not that breastfeeding gives the child a higher IQ.

In the long term, additional schooling doesn’t improve IQ either.

The Flynn Effect: people have been getting smarter since we began to measure IQ, by a rate of 3 points per decade.

Flynn thought that was because the people of today think in a scientific, sometimes abstract manner while they used to think in practical ways in the past.

The Flynn effect shows no signs of stopping.

Chapter 6: Why Is Intelligence So Controversial?

Firstly, because many of the early intelligence scientists were pro-Eugenics. However, the first IQ tests were also designed to help low-IQ kids, not exclude them (Binet).

The reputation of intelligence suffered due to the segregation between high and low-IQ people in British schools, which had a negative effect on the low-IQ people because it didn’t help them increase their social classes. Yet, the fault wasn’t on the IQ tests but on the poor implementation of the system.

Secondly, equality. While equality was the original intent of scientists like Thomson and Burt, it’s equality that’s driving opposition to IQ tests today because we can observe IQ differences across sexes and ethnicities.

Sex-wise, the average IQ of men and women is the same, but their abilities and distribution aren’t.

Women are better at verbal tasks and men are better at spatial abilities; women are located in the middle of the graph (there are more average-IQ women) while men are located at the tail (there are more low and high-IQ men).

Ethnicity (r@c€) is the other extremely taboo finding, so much so that very few researchers dare adventuring themselves in this realm.

So, why study intelligence?

  1. Intelligence is real and we can’t miss this important psychological fact.
  2. There is a link between intelligence and health, so learning about intelligence will help us make people healthier (eg: by targeting different health services according to IQ levels).
  3. Intelligence declines with age, and studying intelligence may help us fix that.
  4. As the world is complexifying, intelligence is increasingly important, which makes boosting IQ an important problem to solve.
  5. Pure scientific curiosity.


Intelligence shouldn’t be so controversial

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