By the time Elliott Jaques published “Death and the Mid-life Crisis” in 1965, the average life expectancy in Western countries had climbed to about 70. It made sense to change your life in your 30s or 40s, because you could expect to live long enough to enjoy your new career or your new spouse.

And it was getting easier to change your life. Women were going to work in record numbers, giving them more financial independence. Middle-class professionals were entering psychotherapy and couples counseling in record numbers and trying to understand themselves. People were starting to treat marriage not just as a romantic institution, but as the source of their self-actualization. Divorce rules were loosening, and the divorce rate was about to surge. And there was dramatic social upheaval, from the civil-rights movement to the birth-control pill. It wasn’t just individuals who had midlife crises. The whole society seemed to be having one, too.

The idea that a midlife crisis is inevitable soon jumped from Jaques’ academic paper to popular culture. And according to the new conventional wisdom, the 40s were the prime time for it to occur. In her 1967 book, The Middle-Age Crisis, the writer Barbara Fried claimed the crisis is “a normal aspect of growth, as natural for those in their 40s as teething is for a younger age group.”