This book is about a simple measurement of the human hand: the 'finger ratio', or the length of the ring finger relative to the index finger. John Manning uses a tiny difference between the sexes - that men tend to have a greater finger ratio than women - to examine a dizzying group of questions about human behaviour, from sexuality, to musical ability, to predisposition to disease. Controversial, but unfailingly clear and balanced, John Manning's cutting-edge research poses many fruitful questions about what makes us as we are.