One number has the power to determine the chances we have, the people we meet, the lives we live. In a competitive world, our performance in IQ tests can shape our destiny. In this, the first popular history of the intelligence test, Stephen Murdoch reveals how universal education, mass immigration into the U.S. in the early 20th century and the demands of mobilisation in the First World War created the need to rank populations by intelligence. In the following decades the tests were used to decide whether people could settle in a new country, whether they could reproduce, even whether they lived or died. While IQ tests have some predictive power, they don't explain people's capacity to think and understand the world around them. What has only ever been a rough guide to ability has, through the seductive power of a single, all-explaining number, come to be seen as an objective and infallible measure of intelligence, even of human merit. Just as bad, we've often tried to reshape society based on exam results alone. Is that the smartest idea anyone ever had?