"In addition, racism which had been viewed primarily as a problem in the Southern states, burst onto the national consciousness following the Great Migration, the relocation of millions of African Americans from their roots in the Southern states to the industrial centers of the North after World War I, particularly in cities such as Boston, Chicago, and New York (Harlem). In northern cities, racial tensions exploded, most violently in Chicago, and lynchings--mob-directed hangings, usually racially motivated—increased dramatically in the 1920s. As a member of the Princeton chapter of the NAACP, Albert Einstein corresponded with W. E. B. Du Bois, and in 1946 Einstein called racism America's "worst disease"."