In the latter half of the 19th century and the first half of the 20th century large numbers of African Americans migrated from the Southern United States to Philadelphia giving the city the largest African American population of a northern U.S. city. In 1896 W. E. B. Du Bois came to Philadelphia and wrote his sociological book "The Philadelphia Negro". Du Bois' study of the city's African Americans found many lived in Center City with growing populations in West Philadelphia. Many African Americans lived in neighborhoods near the rich white families they worked for. There was also a small group of upper class African Americans, both educated, wealthy and Philadelphia born, who descended from freedmen earlier in the century. The aristocratic African Americans were generally isolated, not recognized or even known about by the white population and alienated from the rest of the city's African American population.