In “Caste: The Origins of Our Discontents” (Random House), Isabel Wilkerson contends that the brutal Indian system of hierarchy illuminates more about American racial divides than the idea of race alone can, and early in her book she relays a story that King told about his India trip. He was visiting a school for Dalit children when the principal introduced him as “a fellow untouchable.” The comparison made King flinch—but then its truth overwhelmed him. “In that moment, he realized that the Land of the Free had imposed a caste system not unlike the caste system of India and that he had lived under that system all of his life,” Wilkerson writes. “It was what lay beneath the forces he was fighting in America.”

This story is almost certainly apocryphal, borrowed from a sermon that one of King’s mentors gave more than two decades earlier. In later years, King took little interest in how the idea of caste might apply in his own country. But the anecdote at once lends a civil-rights hero’s weight to Wilkerson’s bold thesis and provides the model response to it: a lightning flash of insight about the mechanics of white supremacy. In her view, racism is only the visible manifestation of something deeper. Underlying and predating racism, and holding white supremacy in place, is a hidden system of social domination: a caste structure that uses neutral human differences, skin color among them, as the basis for ranking human value.

“Caste is insidious and therefore powerful because it is not hatred; it is not necessarily personal,” she writes. “It is the worn grooves of comforting routines and unthinking expectations, patterns of a social order that have been in place for so long that it looks like the natural order of things.” The caste model moves white behavior away from subjective feelings (what motivates these people to do what they do) and into the objective realm of power dynamics (what they do, and to whom). The dynamic that concerns Wilkerson the most is how a dominant caste stops a low-ranking caste from gaining on it.