Toward the close of the nineteenth century in England, a chorus of scientists and intellectuals proclaimed that modern understandings of human evolution and heredity had disproven once and for all the American and French revolutionary doctrine of ‘natural equality’. Full democracy and universal education, however noble as ideals, were essentially against nature, a denial of the ‘vast inequalities’ which stratify the human species. For these belated polemicists against American and French revolutionary ideals, the problem with natural inequality — and the reason it still needed to be defended against latter-day ‘Rousseauists’ – is that it manifests itself most profoundly in an elusive essence: intelligence.

The first practical intelligence test and the concept of IQ were still some years away: it was not until 1905 that the French psychologists Alfred Binet and Théodore Simon would publish their famous intelligence test and, in Simon’s words, ‘secure the idea of human inequality on a basis other than that of a vague impression’.By that time, English scientists and psychologists had been dreaming about a viable test of intelligence for half a century, and the components of the concept of IQ were familiar to the public even if there was no easy shorthand for how they fit together. Over the course of the nineteenth century, a new scientific model of the human mind had emerged. ‘Intelligence’ named the continuity between human and animal minds as well the mind’s capacity to function like a machine. This animal-like and machine-like capacity was the product of evolutionary adaptations, and largely – perhaps overwhelmingly – determined by heredity. It was a measurable entity in the brain (even if no entirely satisfactory measure had yet been found). Intelligence also came to be understood as homogenous ‘general ability’ which varies in quantity but not in kind among people and is thus open to comparison and ranking.