6. What do more intelligent brains look like?

‘What do more intelligent brains look like?’ considers a study that used data from the Lothian Birth Cohort 1936 to test the strength of the correlation between the general intelligence scores of the participants and different measures of their brain’s structure. Magnetic resonance imaging was used to measure total brain volume, brain cortical thickness, brain white matter integrity (or health), and brain white matter hyperintensities. The study showed that people who have higher general intelligence tend to have larger brains, thicker grey matter on the surface of the brain, and healthier white matter brain connections. The associations are not strong, but some aspects of brain structure do relate to intelligence test scores.

4. What are the contributions of environments and genes to intelligence differences?

‘What are the contributions of environments and genes to intelligence differences?’ asks whether genetic inheritance and the environments people experience affect intelligence differences. Researchers use two main resources to answer this question: twins and samples of DNA. Studies of identical and non-identical twins are used to show the contributions of genes, shared environment, and non-shared environment to people’s differences in traits. Twin studies tell us that by adulthood, about two-thirds of intelligence differences are caused by how people vary in their genetic inheritance, and that both shared and non-shared environments make significant contributions to intelligence differences. The introduction of genome-wide association studies in 2011 has provided a new method of estimating the heritability of intelligence.

8. Does intelligence matter for health and longer life?

The new field of cognitive epidemiology looks at whether intelligence tests might have some predictive power in health, and even for death. ‘Does intelligence matter for good health and long life?’ discusses the results of various studies that link the results of intelligence tests at age 11 with medical records in later life. The results show that there is a robust association between higher intelligence test scores in early life and living longer, and having a lower risk of dying from several causes. They also show that, on average, people with higher childhood intelligence have a lower risk of developing various illnesses and are more likely to adopt healthier behaviours as adults.

7. Does intelligence matter in the school and the workplace?

Do intelligence test scores have some predictive power? Can they predict who will do well in achieving more and better educational qualifications, and after school, can they predict who will perform well in the workplace? ‘Does intelligence matter in the school and the workplace?’ looks at the use of the Cognitive Abilities Test in United Kingdom schools and its correlation with GCSE results five years later. The correlation is high: people’s intelligence differences at age 11 are a powerful predictor of their differences in educational outcomes at age 16. In the workplace, the use of general intelligence/psychometric testing in selecting people for work is worthwhile. Intelligence predicts occupational and educational successes.

9. Is intelligence increasing generation after generation?

‘Is intelligence increasing generation after generation?’ discusses the ‘Flynn effect’ of rising IQ. James Flynn noticed that tables of norms for intelligence tests had to be changed every several years. Newer generations were scoring too well on the tests, by comparison with people who were their age some years before. The first response to the Flynn effect suggests that it is real, marking an actual improvement in brain power in successive generations across the 20th century. The second response states that people are not more intelligent, but have become more familiar with the mental tests’ materials. Something in the environment or culture of many countries across the 20th century has led to cognitive ability test scores increasing substantially.

1. Is there one intelligence or many?

‘Is there one intelligence or many?’ considers what is measured in intelligence tests, such as the Wechsler Adult Intelligence Scale IV, and asks whether these different skills are related to each other, or whether they are distinct. Mental abilities, as measured in mental tests, tend to collect together in pools with common associations. So can we talk about a general intelligence? From the various datasets available, we can say with confidence that some people are generally cleverer than others, and also that there is more to human mental ability than just being generally clever. In the 1990s, American psychologist John Carroll produced a three-stratum model of mental abilities to describe a hierarchy of intelligence differences.

10. Do psychologists agree about intelligence differences?

In 1994, The Bell Curve by Richard Herrnstein and Charles Murray was published. It provoked the Western world and the psychological research community into turmoil over the impact that mental ability differences have on people’s destinies. The American Psychological Association (APA) decided that they had a responsibility to put on record some findings about human intelligence differences that attracted broad consensus among psychologists. ‘Do psychologists agree about intelligence differences?’ discusses the report that was drawn up by the APA Task Force, which addressed conceptions of intelligence, intelligence tests and their correlates, the genetic and environmental contributions to intelligence, and group differences in intelligence. The conclusions presented some challenges for future research.

3. Are there sex differences in intelligence?

‘Are there sex differences in intelligence?’ considers the data sets from the Scottish Mental Survey of 1932, the National Longitudinal Survey of Youth from 1979, and the Cognitive Abilities Test 3 sample from 2001–3. All three studies show that there is little evidence of any average difference in intelligence between boys and girls, or young adults. However, for overall general intelligence, there are slightly more girls than boys in these samples around the average scores, and proportionately more males than females at the higher and lower extremes. Among the cognitive domains, a different pattern occurred for verbal reasoning: there were more girls at the higher scores.

5. Are smarter people faster?

A long-standing idea is that people who do well on intelligence tests might be better at some basic psychological processes, such as mental speed, which is also called processing speed. Smarter people might have faster brains, and their intelligence might follow from that. ‘Are smarter people faster?’ considers the West of Scotland Twenty-07 study, which reported on the correlation between reaction time and intelligence, and the Lothian Birth Cohort of 1936, which measured processing speed using inspection time. These studies show medium to strong levels of correlation between choice reaction time and general intelligence and that people with higher intelligence tended to make more correct decisions about the inspection time stimuli.

2. What happens to intelligence as we grow older?

The study of cognitive ageing is arguably one of the most lively, exciting, and important in the field of human intelligence, as the proportion of older people in the population grows larger and as people live longer. ‘What happens to intelligence as we grow older?’ considers Salthouse’s Virginia studies and the Scottish Mental Surveys of 1932 and 1947. People differ in how much their intelligence changes from youth to older age, but why does some people’s intelligence age better than others? Age wears away at some cognitive domains and leaves others largely intact. With ageing, an important distinction emerges between those stratum II abilities that are part of so-called ‘fluid’ and ‘crystallized’ intelligence.