Thomas Sowell's research highlights early 20th-century anxieties about "racial degeneration" caused by immigrants from Southern and Eastern Europe. Writers like Kenneth L. Roberts and George Lorimer argued that only "Nordic" types were fit for American citizenship, while Jews, Italians, and Slavs were viewed as "mongrel races." This history illustrates the rigid "color line" that prevents transracial identity from being accepted in the same way as transgender identity.