Why spy agencies, IQ tests and AI all use the same word

Francis Galton made one of the first attempts to measure people’s faculties in bulk. At the International Health Exhibition in London in 1884, his Anthropometric Laboratory measured the bodies and senses of 9,337 visitors. He warned, on the same page as the total, that they “were not random specimens of the crowd”. Galton believed ability was inherited; his 1869 book Hereditary Genius set out to show it.

In October 1904 the French Minister of Public Instruction set up a commission to decide what should be done for children who could not keep up at school. It ruled that no child could be moved to a special class without an examination, and left the method to others: it had been formed “to do a work of administration, not a work of science”. Alfred Binet and Théodore Simon supplied the method in 1905: a series of tasks of increasing difficulty, from following a lit match with the eyes to explaining “what difference there is between weariness and sadness”. In 1908 they revised it so that each task sat at the age by which most children could do it.

Binet was careful about what his scale did. It “does not permit the measure of the intelligence”, he wrote, “because intellectual qualities are not superposable”. It ranked children; only “for the necessities of practice”, he added, was that ranking “equivalent to a measure”.

In 1909, in Modern Ideas about Children, Binet attacked the philosophers who had lent their support to such “deplorable verdicts” by claiming “that an individual’s intelligence is a fixed quantity, a quantity that cannot be increased. We must protest and react against this brutal pessimism.”

His successors took the other side. In 1912 the German psychologist William Stern proposed dividing the “mental age” a test gives by the child’s real age. He called the result the Intelligenzquotient and wrote it as a fraction: an eight-year-old with a mental age of six had an IQ of 6/8, or 0.75. He intended it for children already labelled feeble-minded. In 1916 Lewis Terman, at Stanford, set the average at “100 per cent”, printed IQs as whole numbers and drew bands: above 140, “near genius or genius”; below 70, “definite feeble-mindedness”. On the same page he admitted that “the boundary lines between such groups are absolutely arbitrary”.

When the United States entered the First World War, a team of psychologists led by Robert Yerkes persuaded the army to test its recruits. By 31 January 1919, 1,726,966 men had taken the Army Alpha, a written test, or the Army Beta, a pictorial one for those who could not read English.

What g is, and what it is not

g is a statistical summary: what a set of tests has in common. In modern test batteries it usually accounts for roughly 40 to 50 per cent of the differences between people. The rest belongs to more specific abilities. Raymond Cattell separated fluid intelligence, reasoning with new material, from crystallised intelligence, knowledge already learned. John Carroll’s reanalysis of more than 460 data sets, published in 1993, arranged the abilities in three layers with g at the top, and many current test batteries are mapped onto that model.

Spearman overreached from his own data. He concluded that a “general sensory discrimination”, the ability to tell apart tones, shades of light and weights, corresponded with general intelligence to a degree “not appreciably less than absolute”. The evidence came from 123 children and adolescents in a few English schools.

Differences in g are partly inherited. In twin studies, the share of the variation explained by genes rises from about 20% in infancy to about 40% in adolescence and 60% in adulthood. Heritability describes differences within a population. It does not say how much of any one person’s intelligence is fixed, which was Binet’s point.

In 1984 James Flynn, a political scientist working in New Zealand, compared the old and new norms of American tests and found a rise of 13.8 IQ points between 1932 and 1978, about 0.3 a year. Three years later he found gains of 5 to 25 points in a single generation across 14 countries.

Flynn drew a striking conclusion: the tests do not measure intelligence itself but “a correlate with a weak causal link to intelligence”. The largest gains were on tests of reasoning with unfamiliar material, the kind Cattell called fluid, and the smallest on vocabulary and knowledge.

In Denmark and Norway the rise stopped. In Denmark, among more than half a million young men tested between 1959 and 2004, the gain after 1989 was small and had disappeared by 2000. In Norway, conscription scores fell from the 1975 cohort on, and the fall shows up between brothers: the cause is in the environment, not in the genes.

In 1974 Alan Baddeley and Graham Hitch replaced the idea of a single short-term store with a “work space” of limited size, shared between holding things and doing things with them. In 2001 Nelson Cowan put its capacity at about four chunks. George Miller’s famous “seven, plus or minus two” was, in Cowan’s words, more “a rhetorical device” than a real limit.

In 1990 Patrick Kyllonen and Raymond Christal tested military recruits and found correlations of 0.80 to 0.90 between working memory and reasoning. They titled the paper with a question: Reasoning ability is (little more than) working-memory capacity?!

The answer depends on how you measure. Averaging many kinds of test across 86 samples, Phillip Ackerman and his colleagues found a correlation of 0.48 in 2005: less than a quarter of the variance shared. Michael Kane and his colleagues, pooling fourteen studies designed for the question, found 0.72, about half. Both sides agreed that working memory and intelligence are related and are not the same thing.