In his 19 years as a law professor at UCLA, Richard Sander has pondered a nagging question: Does affirmative action help or hinder African Americans who want to become lawyers?

Two years ago, he published research suggesting that racial preferences at law firms might be responsible for black lawyers’ high rate of attrition and difficulty making partner. He hypothesized that in the interest of promoting diversity, law firms sometimes hired black lawyers who were underqualified, and that when there was a “credentials gap” between black and white lawyers at a firm, black lawyers often were less likely to advance and more likely to leave the firm.