In 1994, two sociologists went to Red Hook, Brooklyn, to solve a mystery. Red Hook abutted the East River, and along the waterfront sat shipping companies and warehouses — all in need of low-skilled labor. Next door sat a housing project teeming with exactly that. But the locals — primarily African Americans — didn't get hired. Instead, the jobs went to workers from outside the neighborhood, often Caribbean immigrants. Employers, wrote The New Yorker's Malcolm Gladwell in summarizing the sociologists' findings, "had developed an elaborate mechanism for distinguishing between those who they felt were 'good' blacks and those they felt...