If you are exasperated by the parlous state of public discourse and debate - in politics, popular culture, or society generally - then you are not alone. Writers Helen Razer and Bernard Keane are similarly cranky about ‘the decline of reason’ and the ascendancy of ‘stupid’ ideas. In their new book, A Short History of Stupid, they take aim at anti-scientific thinking, government paternalism, the war on terror, contemporary feminism, and more. Paul Barclay talks to them.