The word "happiness," of course, appears in one of the most famous sentences in U.S. history: "We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness." But at some point, that crucial bit of hedging contained in the words "pursuit of" seemed to dissolve, and left us with a culture that views abiding happiness as a birthright. It's hard to pinpoint exactly when this happened. Though it was certainly visible to the British social theorist Harriet Martineau, who in her 1838 book Retrospect of Western Travel, described "the inexhaustible American mirth" and noted of Americans that, "One of the rarest characters among them, and a great treasure to all his sportive neighbours, is a man who cannot take a joke." In the history of this country, "the feeling, the imperative… to feel happy is deeply rooted historically and pervasive," says Daniel Horowitz, emeritus professor of American Studies at Smith College and author of the book Happier? The History of a Cultural Movement That Aspired to Transform America.