What Is Intelligence and How Is It Measured?

Intelligence is one of those words almost everyone uses but few people can pin down. It has been picked over by psychologists for more than a century, and the field still has no single definition that commands agreement.

In everyday speech, intelligence usually refers to the ability to learn, reason and apply what you know. It covers things like solving problems, thinking in the abstract, picking up new skills quickly and adapting to unfamiliar situations. Some researchers put more weight on judgement and understanding complex ideas. Others focus on creativity, memory or mental speed. Most agree that it is not one single skill but a cluster of related abilities that tend to go together.

Adapting well to your surroundings is often treated as a sign of intelligence. Surroundings here means more than the physical environment. It includes the people around you, whether that is family, colleagues, classmates or strangers. Handling social situations sensibly is part of the picture, not separate from it.

How intelligence is measured

Measurement is just as contested as definition. The most widely recognised tool is the IQ test, which stands for intelligence quotient. A typical test sets a range of tasks covering verbal reasoning, numerical reasoning, spatial awareness, logic and short term memory. Your score is compared against the results of a large sample of people of similar age.

When you plot the scores of a big enough group, they form the familiar bell shaped curve. Most people sit somewhere in the middle, with smaller numbers trailing off at the high and low ends. An average score is set at 100 by design, and the scoring is adjusted over time so that the average stays there even as test performance changes across generations.

IQ tests are used in education, occupational assessment and clinical work. In the UK, educational psychologists working through local authorities or privately may use tests such as the Wechsler scales when assessing children for learning difficulties or giftedness. In practice these tests form part of a wider assessment that takes in classroom observation, interviews with parents and teachers, and a look at the child's developmental history. A single score on its own is rarely treated as the final word.

General intelligence, or 'g'

The idea that there is a single underlying mental ability was put forward by the English psychologist Charles Spearman in the early 1900s. He noticed that people who did well on one type of mental task tended to do well on others, and called this shared factor general intelligence, or 'g'.

Later brain imaging work has given some support to the idea. Activity in the lateral prefrontal cortex, near the front of the brain, tends to rise when people tackle demanding reasoning problems. Even so, plenty of researchers think 'g' is too tidy an answer. Critics point out that it tells you little about why people differ, and that education, health, nutrition and opportunity all shape test performance. Two people with similar raw ability can end up with very different scores depending on their schooling and background.

More than one kind of intelligence

Dissatisfaction with the single score approach led to theories of multiple intelligences. The best known belongs to the American psychologist Howard Gardner, who suggested that human ability breaks down into several fairly independent areas. His list includes logical, linguistic, spatial, musical, bodily or kinaesthetic, interpersonal, intrapersonal and naturalist intelligence. Gardner's argument is that standard IQ tests mostly measure the first two or three and ignore the rest.

Robert Sternberg took a different route. He proposed three sides to intelligence: analytical, which is the kind tested in exams, practical, meaning common sense and knowing how to get things done, and creative, meaning the ability to come up with new ideas. In his view, someone who is strong in only one of these may struggle in real life despite scoring well on paper.

Gardner's framework has been popular in schools, though it is fair to say it has not been firmly established by experimental evidence. It is more a useful way of thinking about ability than a proven scientific model.

Emotional intelligence, often shortened to EQ, became widely known through the writing of the journalist and psychologist Daniel Goleman in the 1990s. It describes how well you recognise your own feelings, manage them, pick up on other people's emotions and handle relationships. Employers sometimes include it in recruitment and development work because it links to teamwork, leadership and dealing with customers. The evidence for EQ as a distinct and measurable trait is mixed, and there is ongoing debate about how best to assess it, but few would argue that social and emotional skills are unimportant in work or daily life.

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Intelligent, clever or just well informed?

In schools and universities, intelligence often gets confused with academic results. The two overlap but they are not the same. Exam performance depends on teaching, effort, motivation, the support a student gets at home and plenty of other factors alongside raw ability. Someone can know a great deal without being especially quick at reasoning, and someone can be quick witted without having had the chance to learn much.

The word intelligence comes from the Latin intellegere, meaning to understand. Understanding is not quite the same as being smart, which tends to suggest speed, or clever, which often implies a creative or inventive turn of mind. In everyday English these words are used loosely and more or less interchangeably, but they do pick out slightly different qualities.

What to take from all this

Intelligence is real enough to measure, at least partially, but too broad to sum up in a single number. IQ scores can be useful for particular purposes, such as identifying a child who needs extra support at school or investigating a suspected cognitive condition. They are a poor guide to someone's worth, character or likely success in life.

If you are considering a formal assessment for yourself or a family member, speak to your GP or, in the case of a child, to the school's special educational needs coordinator, usually known as the SENCO. The SENCO can discuss concerns, arrange in-school support and, where appropriate, request involvement from the local authority's educational psychology service. Waiting times for local authority assessments can be long, and the criteria for referral vary between areas, so it is worth asking early on what is likely to be available.

Private assessment is another option, though it comes at a cost that can run into several hundred pounds or more depending on the scope of the work. Whether the assessment is private or through a local authority, a good report will set the scores in context, explain what they do and do not mean, and suggest practical next steps rather than simply handing you a number.