Histories of masculinity have documented changes in the definition of American masculinity across centuries, but not decades. This article examines prescriptive and proscriptive statements about men's behavior as they appeared in William Randolph Hearst's American Weekly during the interwar period (1918–1940). Written by professionals and providing summaries of current research, the American Weekly provided authoritative accounts of masculinity to a mass audience. In the early 1920s, reason was explicitly masculine and central to the construction of masculinity, and gender differences were routinely attributed to men's superior evolution. By the late 1930s, reason was implicitly masculine, the breadwinner role was central, and gender differences relied on the known, but unstated, superiority of men.