Before the nineteenth century, the term normal was rarely ever associated with human behaviour. Normal was a term used in maths: people weren't normal - triangles were. But from the 1830s, this branch of science really took off across Europe and North America, with a proliferation of IQ tests, sex studies, a census of hallucinations. This book tells the surprising history how the very notion of the normal came about, how it shaped us all, often while entrenching oppressive values.