Before the 1960s, it was really hard to get divorced in America. Typically, the only way to do it was to convince a judge that your spouse had committed some form of wrongdoing, like adultery, abandonment, or “cruelty” (that is, abuse). This could be difficult: “Even if you could prove you had been hit, that didn’t necessarily mean it rose to the level of cruelty that justified a divorce,” said Marcia Zug, a family law professor at the University of South Carolina. Then came a revolution: In 1969, then-Gov. Ronald Reagan of California (who was himself divorced) signed the nation’s...