Jaques didn’t claim to be the first to detect this midlife change. He pointed out that, in the 14th century, Dante Alighieri’s protagonist in The Divine Comedy—who scholars say is 35—famously declares at the beginning of the book, “Midway upon the journey of our life / I found myself within a forest dark / For the straightforward pathway had been lost.”

But Jaques offered a modern, clinical explanation, and—crucially—he gave the experience a name: the “mid life crisis.”

As he addressed the meeting in London, Jaques was nervous. Many of the leading psychoanalysts of the day were sitting in the audience, including the society’s president, Donald Winnicott, renowned for his theory of transitional objects, and Jaques’ own mentor, the famed child psychologist Melanie Klein.

It was an acrimonious group, which had split into competing factions. Attendees were known to pounce on presenters during the questioning period. And Jaques wasn’t just presenting an abstract theory: He later told an interviewer that the depressed 36-year-old patient he described in the paper was himself.

When he finished reading the paper, titled “The Mid Life Crisis,” Jaques paused and waited to be attacked. Instead, after a very brief discussion, “there was dead silence,” he recalled later. “Which was very, very embarrassing, nobody got up to speak. This was new, this is absolutely rare.” The next day, Melanie Klein tried to cheer him up, saying, “If there’s one thing the Psychoanalytic Society cannot cope with, it’s the theme of death.”

Chastened, Jaques put “The Mid Life Crisis” aside. He went on to write about far less personal topics, including a theory of time and work. “I was certainly utterly convinced that the paper was a complete failure,” he recalled.

But he didn’t forget how it felt to be that troubled man standing on the crest of the hill. About six years later, he submitted the paper to The International Journal of Psychoanalysis, which published it in its October 1965 issue under the title “Death and the Mid-life Crisis.”

This time, instead of silence, there was an enormous appetite for Jaques’ theory. The midlife crisis was now aligned with the zeitgeist.

If you were a man born in 1900, you had only about a 50 percent chance of living to age 60. The average life expectancy for men was around 52. It was fair to think of age 40 as the beginning of the end.

But life spans in rich countries were increasing by about 2.3 years per decade. Someone born in the 1930s had nearly an 80 percent chance of living until age 60. That gave age 40 a new vitality. Life Begins at Forty was the best-selling American nonfiction book of 1933. Walter Pitkin, the journalist who wrote it, explained that “before the Machine Age, men wore out at forty.” But thanks to industrialization, new medicines, and electric dishwashers, “men and women alike turn from the ancient task of making a living to the strange new task of living.”