A mystery novel, a history book, and a fantasy epic may have little in common in plot or style. But count the words inside them and a strange regularity appears: many new words show up early, then fewer and fewer as the author reuses what has already been introduced. That pattern, known as Heaps' law, turns out not to belong to books alone. A new study published in the Proceedings of the National Academy of Sciences finds that the same rule also describes how many complex systems grow, from living cells and corporations to universities and government agencies—and could even be used to predict how they will change in the future.
