The intelligence of that creature known as a crowd is the square root of the number of people in it.

Pratchett’s line is a comic, pseudo-mathematical way of expressing a long-standing idea in social psychology: individuals in groups often behave less thoughtfully than they would alone. By saying a crowd’s “intelligence” scales only with the square root of its size, he implies diminishing returns—adding more people does not add proportionate wisdom, and may even amplify confusion, conformity, and emotional contagion. The phrasing (“that creature known as a crowd”) personifies the crowd as something with its own instincts, distinct from its members. The joke lands because it mimics scientific precision while delivering a satirical verdict on mob behavior and the fragility of rationality in mass settings.