Ultimately, in fact, it may simply not be possible to say why the Mona Lisa is the most famous painting in the world or why the Harry Potter books sold more than 350 million copies within ten years, or why Facebook has attracted more than 500 million users. In the end, the only honest explanation may be the one given by the publisher of Lynne Truss’s surprise bestseller, Eats Shoots and Leaves, who, when asked to explain its success, replied that “it sold well because lots of people bought it.”

It may not surprise you to learn that many people do not particularly like this conclusion. Most of us would be prepared to admit that our decisions are influenced by what other people think—sometimes, anyway. But it’s one thing to acknowledge that once in a while our behavior gets nudged this way or that by what other people are doing, and it’s quite another to concede that as a consequence, true explanations for the success of an author or a company, unexpected changes in social norms, or the sudden collapse of a seemingly impregnable political regime may simply lie beyond our reach.