Forty-seven years ago the musical Hair opened on Broadway. Elderly mavens — the core theater audience then, unlike the tourists thronging Disney-movie-knockoffs today — were instructed that America was entering an “Age of Aquarius.” The old moral rules were extinct: we were entering a new era of freedom, experimentation, self-expression.

In some ways Hair’s prediction came true. Rates of divorce, cohabitation before marriage and illegitimate births rose sharply in the years after 1968. The percentages of children living with two parents fell sharply. Hair’s version of history — hundreds of years of oppression suddenly followed by a sudden trend to ever more liberation — seemed plausible, even persuasive.