Hayes was influenced by Charles A. Beard, a proponent of the "New History", which emphasized the importance of cultural economic developments as opposed to just warfare and diplomacy. Hayes argued the New History demonstrated that Original Sin was integral to human existence. His two-volume "Political and Cultural History of Europe", long a major textbook, is filled with examples of such thought, none more so than his discussion of the Industrial Revolution in England. Hayes also developed the historical interpretation of nationalism and was known as the Father of Nationalism, inspiring many students to research in this field. He was an advocate of democratic social reform. His research interests shifted after 1914, and increasingly focused nationalism, which he denounced as one of history's great evils; he said that along with imperialism and militarism, nationalism had caused World War I. Hayes was an internationalist, unlike Beard, and opposed isolationism in the 1930s while denouncing totalitarianism.