In March 1946, scientists began to track thousands of children born in one cold week. No one imagined that this would become the longest-running study of human development in the world, growing to encompass five generations of children. Today, they are some of the best-studied people on the planet, and the simple act of observing human life has changed the way we are born, schooled, parent and die. This is the tale of these studies and the remarkable discoveries that have come from them. Touching people across the globe, they are one of the world's best-kept secrets.