Humans have been measuring each other's intelligence for a long time. In China during the Xi Zhou dynasty (1046 to 771BC), candidates for official positions were formally tested on a range of criteria including the “six skills”: arithmetic, archery, horsemanship, music, writing and the performance of rituals and ceremonies. The notion of a universal, objective scale of intelligence, however, did not take shape until the 19th century and the arrival of Darwinism. If Charles Darwin is the father of modern biology, then the father of modern intelligence testing is his cousin, Francis Galton – statistician, polymath and founder of eugenics. In 1884, he set up an “anthropometric laboratory” at the International Health Exhibition in London, and measured, among other things, the reaction times, eyesight, colour sensitivity and steadiness of hand of more than 9,000 men and women as he looked for links between their physical and mental characteristics.
Searching for genius, Galton failed to develop a working intelligence test. That was left to a French psychologist, Alfred Binet, and his student, Victor Henri. Binet was commissioned to study the large numbers of poor children in the city's asylums and to find out whether they were mentally incapacitated or simply untaught. His resulting 1904 test of 30 indicators – from the eye being able to follow a lit match, to memory and vocabulary questions – provided the basis of modern intelligence testing. In 1916, Lewis Terman, a professor of psychology at Stanford University, revised and expanded the test, creating the Stanford-Binet IQ test, which is still used today. Although more moderate than many of his contemporaries, Terman adhered to the social Darwinism of his time – in 1930, 24 US states had sterilisation laws – and he had hopes for the social potential of his work. “This,” he wrote in 1919, “will ultimately result in the curtailing of the reproduction of feeble-mindedness.”
Intelligence testing has proved contentious ever since. In the US, where more than nine million men underwent various forms of IQ and ability tests during the second world war, the enthusiasm for testing has been matched only by the ferocity of arguments over what exactly it proves. IQ tests for children, the SAT Reasoning Test for college applicants and psychometric testing by companies may have been designed with the goal of identifying individual talent, but often their larger consequence has been to highlight differences already inherent in society. Variations between the sexes and ethnic groups have led to toxic arguments about bias and inequality and power: who gets to define intelligence? Who designs the tests? In its various iterations, the debate about IQ testing in the US normally returns to the persistent, albeit shrinking, lag between results for white and black populations.