In “Formal Theories of Mass Behaviour”, William McPhee noted that a disproportionate share of the audience for a hit was made up of people who consumed few products of that type. (Many other studies have since reached the same conclusion.) A lot of the people who read a bestselling novel, for example, do not read much other fiction. By contrast, the audience for an obscure novel is largely composed of people who read a lot. That means the least popular books are judged by people who have the highest standards, while the most popular are judged by people who literally do not know any better. An American who read just one book this year was disproportionately likely to have read “The Lost Symbol”, by Dan Brown. He almost certainly liked it.

This explains why bestselling books, or blockbuster films, occasionally seem to grow not just more quickly than products which are merely very popular, but also in a wholly different way. As a media product moves from the pool of frequent consumers into the ocean of occasional consumers, the prevailing attitude to it—what Hollywood folk call word of mouth—can become less critical. The hit is carried along by a wave of ill-informed goodwill.

—The Economist

[It also explains why the fact people who haven’t  read a book since high school are enthusiastic about your manuscript isn’t persuasive to editors.]