Robert Louis Stevenson's terrifying story of an urbane doctor who turns into a homicidal madman sold an unprecedented million copies before the turn of the twentieth century. One hundred years later, the expression 'Jekyll and Hyde personality' still characterises a person, often schizophrenic, at war with his darker side. Basing their work on impressive new research, McNally and Florescu shine a light into the mystery of why this image still rings true, showing that The Strange Case of Dr Jekyll and Mr Hyde is more than just a ripping good yarn: it is an insight into human nature and the true character of evil.