He pioneered social history and urban history. He was a Progressive Era intellectual who stressed material causes (like economic profit) and downplayed ideology and values as motivations for historical actors. He was highly influential as a director of PhD dissertations at Harvard for three decades, especially in the fields of social, women's, and immigration history. He commented in 1922, "From reading history in textbooks one would think half of our population made only a negligible contribution to history." He promoted social history by co-editing the 13-volume "History of American Life" (1928–1943) series with Dixon Ryan Fox. These thick volumes, written by leading young scholars, mostly avoid politics, individuals, and constitutional issues. They instead focus on such topics as society, demographics, economic, housing, fashion, sports, education, and cultural life.