Isabel Wilkerson spent 10 years and interviewed more than 1,500 people to write The Warmth of Other Suns: The Epic Story of America's Great Migration, her magnificent opus on the flight of 6 million African Americans from the South to the North and West. Wilkerson has challenged long-standing assumptions that blacks moved simply for economic opportunity.

In fact, she says, it was a political act, a defection driven by "the overarching desire was to escape a caste system that controlled their lives from the moment they awoke in the morning to the time they went to sleep." Gates, who is the Alphonse Fletcher University Professor and the director of the W.E.B. Du Bois Institute for African and African American Research at Harvard University, interviewed Wilkerson in Cambridge, Mass. See their interview in the clips below.