Most of us who grow up in the United States learn a reassuring narrative of ever-expanding tolerance. Yes, the country’s birth was tainted with the original sin of slavery, but Lincoln freed the slaves, the Supreme Court desegregated schools, and we finally elected a black president. The Founding Fathers may have all been men, but in their wisdom they created a constitution that would later allow women to gain the vote. And now the legal definition of marriage has broadened to include gays and lesbians. We are, it appears, an increasingly inclusive nation. But a parallel, much darker river runs through American history.