Intelligence and IQ: How Bright Are You?

Psychology 101: The 101 Ideas, Concepts and Theories that Have Shaped Our World - Adrian Furnham 2021

Intelligence and IQ: How Bright Are You?

The prime author and mover of the universe is intelligence. (St Augustine, Confessions)

Intelligence is quickness to apprehend as distinct from ability, which is capacity to act wisely on the thing apprehended. (A.N. Whitehead, Dialogues, 1930)

To judge well, to comprehend well, to reason well. These are the essential activities of intelligence. (A. Binet and T. Simon, The Intelligence of the Feeble Minded, 1916)

Intelligence is really a kind of taste: taste in ideas. (Susan Sontag, Against Interpretation, 1966)

Intelligence testing has had a chequered and controversial history in psychology. Yet research from many scholars over many areas in many countries and over a century has demonstrated the pervasive influence of intelligence in all walks of life from health and happiness to wealth and welfare. It is a basic building block for the differential psychologist. It is, quite simply, the most easily and reliably measured individual difference variable with the best reliability and validity in the whole of psychology.

The last 10—20 years have seen different attempts to expand the concept of intelligence to devise tests of greater face validity and acceptability, to monitor group (sex, race) differences carefully, and provide incontrovertible evidence of the predictive validity of tests.

There are many fundamental psychometric questions about intelligence which get asked again and again:

Do intelligence tests have good reliability? Yes. Only high anxiety or situationally-induced low motivation causes test-retest correlations to drop below a correlation of r = .90

Are IQ scores stable over (lifetime)? Yes. By late childhood tests reasonably and accurately predict adult scores as many as 50 years later.

Do intelligence tests have adequate validity? Yes. They predict school success (around r = .50) and how long people remain in school (r = .70), and many other educational, or organizational and social variables.

Do intelligence tests predict job performance and academic success? Yes. There are various caveats to this question which are central to this chapter.

But are there not multiple intelligences? No. Not in the sense that most people are very good at some cognitive tasks and very bad at others. Generally, we find that scores on all sorts of (good) IQ tests correlate positively and significantly with one another. That is, people perform at a broadly similar level across all tasks (vocabulary, maths, etc.).

Are all IQ tests equally good? No. It takes quite some effort to develop, refine and produce a test that gives an all-round picture of a person’s cognitive functioning.

Other questions include: How much is intelligence shaped by environment vs. heredity? How much can intelligence be enhanced?

The publication of a recent highly controversial book on intelligence (The Bell Curve, Herrnstein & Murray, 1994) began a passionate, although not necessarily well-informed, debate led by over 50 of the world’s experts on intelligence. Their summary is an excellent and clear statement on what psychologists think about intelligence. These are the first six points:

1 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 descriptive of test-taking smarts.

2 Intelligence, so defined, can be measured, and intelligence tests measure it well. They are among the most accurate (in technical terms, reliable and valid) of all psychological tests and assessments. They do not measure creativity, character, personality or other important differences among individuals, nor are they intended to.

3 While there are different types of intelligence tests, they all measure the same intelligence. Some use words or numbers and require specific cultural knowledge (such as vocabulary). Others do not, and instead use shapes or designs and require knowledge of only simple, universal concepts (many/few, open/closed, up/down).

4 The spread of people along the IQ continuum, from low to high, can be represented well by the bell curve (in statistical jargon, the ’normal curve’). Most people cluster around the average (IQ 100). Few are either very bright or very dull: about 3 per cent of Americans score above IQ 130 (often considered the threshold for ’giftedness’), with about the same percentage below IQ 70 (IQ 70—75 often being considered the threshold for mental retardation).

5 Intelligence tests are not culturally biased against African-American or other native-born, English-speaking people in the USA. Rather, IQ scores predict equally accurately for all such Americans, regardless of race and social class. Individuals who do not understand English well can be given either a non-verbal test or one in their native language.

6 The brain processes underlying intelligence are still little understood. Current research looks, for example, at speed of neural transmission, glucose (energy) uptake and electrical activity of the brain.


It was an American political scientist working in New Zealand, James Flynn, who gave his name to this ’effect’. He noticed that every so often the norms which describe typical scores for different age, sex and race groups had to change. Every few years scores in the same age group showed that differences were growing. In short, people were doing better over time. The tests seemed to be getting easier or we were, as a species, getting brighter… or both. This means a good score in 1990 was a brilliant score in 1970 but only an average score in 2015.

Was this effect true of many countries and many tests? The answer was YES. There seemed to be impressive evidence of ’massive IQ gains’. But the central question became, why? Flynn never questioned the reliability, validity and usefulness of IQ tests in educational and occupational settings.


Education: In most countries, with every generation people are spending longer at school and with better facilities. Schooling is compulsory and people from all backgrounds are used to learning and being tested. Intelligence is related to learning, so as education is better, more widespread scores get higher.

Nutrition: People are now better nourished, particularly in childhood, which reduces the incidence of ’backwardness’ in the population. There are fewer people who had poor nutrition in their youth so the bottom end of the distribution is removed. Which means the average score goes up.

Social trends: We are all now much more used to timed tests and performing against the clock. People are familiar with tests and testing and so do better overall.

Parental involvement: The idea is that parents provide richer home environments for their children and express a greater interest in their education than they used to. They have higher expectations and get involved more. The trend to have smaller families, where parents invest more in their children, may also be an important factor.

Social environment: The world is more complex and stimulating. Modernization and new technology means people have to manipulate abstract concepts more, which is essentially what intelligence tests measure.


Deary, I. (2001). Intelligence: A very short introduction. Oxford: Oxford University Press.

Eysenck, H. (1998). Intelligence: A new look. London: Transaction Publishers.

Flynn, J. (1987). Massive IQ gains in 14 nations. Psychological Bulletin, 101, 171—91.