In 1904, English psychologist Charles Spearman discovered there was a strange link between certain mental skills, such as mathematics, verbal fluency, spatial visualisation and memory.
He found that people who did well at one tended to do well at others, while those who did poorly at one would do poorly at others. And his finding has been shown to be true so many times that it has been called “arguably the most replicated result in all psychology”.
By using statistics, it’s possible to derive a single factor, known as general intelligence (g), that indicates the level of general cognitive ability compared to other people. This general intelligence has been divided into two types.
The first is fluid intelligence (gf), which doesn’t depend on formal education and is more about solving abstract reasoning problems. The second, crystallised intelligence (gc), is more about learned experience, general information and vocabulary.
Researchers claim that gf peaks at around age 20 and then declines, while gc stays stable or improves with age. It’s known that g is heritable – you’re likely to inherit mental skills from your parents.
We can estimate general intelligence (g) using intelligence quotient (IQ) tests. These are standardised tests that provide highly reliable results. This means that even if a person does several different IQ tests, they’re likely to achieve much the same score each time.
There are many kinds of IQ tests, focusing on different cognitive abilities, but all rely on the fact that if you score well in one kind of mental task you’re likely to score well in others.
The raw IQ score is adjusted using statistics to ensure that about 66 per cent of people will score between 85 and 115, and 2.5 per cent will score more than 130 or less than 70. Raw scores have been rising over the decades, however. One study showed that British children’s raw IQ scores rose 14 points from 1942 to 2008.
IQ tests have been shown to be predictive of job performance, income, social status, nutrition and even mortality. But despite their reliability, they’ve had a torrid history.
Some of the originators became involved in eugenics, believing high IQ could be bred into populations. It still has fierce critics.
IQ is measuring one aspect of our brains, but there are other kinds of intelligence, such as emotional intelligence or rational thought, where you’re able to make good judgements and decisions.
Some liken IQ to measuring the brightness of a searchlight – it may be bright but pointed in a silly direction it illuminates nothing of value. Many people with high IQs are successful, but not everyone with high IQs will be sensible, rational or make good choices in life.
This article is an answer to the question (asked by William Rawlings via email) 'How do IQ tests work?'
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