In 1993, New York’s Democratic senator Daniel Patrick Moynihan, a former social scientist, made an incisive observation: Humans have a limited ability to cope with people behaving in ways that depart from shared standards. When unwritten rules are violated over and over, Moynihan observed, societies have a tendency to “define deviancy down”—to shift the standard. What was once seen as abnormal becomes normal. Moynihan applied this insight, controversially, to America’s growing social tolerance for single-parent families, high murder rates, and mental illness.
