When the late Richard Herrnstein and I published The Bell Curve eleven years ago, the furor over its discussion of ethnic differences in IQ was so intense that most people who have not read the book still think it was about race. Since then, I have deliberately not published anything about group differences in IQ, mostly to give the real topic of The Bell Curve—the role of intelligence in reshaping America’s class structure—a chance to surface. The Lawrence Summers affair last January made me rethink my silence. The president of Harvard University offered a few mild, speculative, off-the-record remarks about...