What is the History of the "Midlife Crisis"? Where and when did it originate? How and to what extent did it spread from the culture that created it? Is it purely a modern phenomenon, or does it have much older roots?

Midway upon the journey of our life

I found myself within a forest dark,

For the straightforward pathway had been lost.

- Inferno, Canto I - Dante Alighieri

The initial aspect of this question is actually quite straightforward, the term "mid-life crisis" entered into the professional jargon of psychology in 1957 with a reading of a paper by Elliott Jaques at a meeting of the British Psycho-Analytical Society. In this reading, Jaques claimed that people in their mid-30s tend to undergo a depressive episode (i.e., a period of depression lasting at least two weeks) as they reflect upon their own morality and what they have accomplished in life. This initial reading was actually poorly received. Jaques got one of the worse receptions that an academic can get, silence:

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.”

As a result of this poor reception, it wouldn't be until 1965 that Jaques would work the paper into a published manuscript which happened to catch the public at just the right time for the term to jump from professional jargon to part of the cultural zeitgeist.

If we step back, the underlying concept that people reflect upon their mortality, accomplishments, and goals isn't new; as evidenced by Dante Alighieri in the opening to the Divine Comedy. However, whereas a white person born in the U.S. in 1900 might have had a life expectancy at birth of about 50,^(1) rising to about 60 if they survived to 10, by the 1960s, most people alive could reasonably expect to make it to their mid-60s, possibly even their early-70s. So people where living longer, meaning that people were working longer, living with the same spouse longer, and living without children longer. In short, the "normal" progression of the major life milestones of youth, early adulthood, marriage, children, and eventual death had been disrupted.^(2) Inquires into these changes had already started with the 1933 with the publications of Life Begins at Forty by Walter B. Pitkin followed by the 1935 comedy film of the same name, although the academic response was more muted given the perspective that living longer was not seen to trigger a crisis.

However, the post-war era saw an increase in the empty nests (i.e., all children have moved out), increasing wealth, and more labor saving technologies; allowing more time to reflect upon what it all meant. Couple this with the dramatic social changes of the 1960s (i.e., women's liberation movement, civil rights, Vietnam War, etc.) and a backdrop in which divorced laws were being loosened, and people were generally seeking new meaning in their lives. In 1967, Barbara Fried published the book The Middle-Age Crisis which introduced to the general public the idea that you should expect to a crisis to occur in your 40s as your body changed, you career plateaued (or peaked), your children left the home, and nothing is left but decline.^(3) This idea of the midlife crisis was reinforced by the government, as noted in a special task force report on "Work in America":

A general feeling of obsolescence appears to overtake middle managers when they-reach their late 30's. Their careers appear to have reached a plateau, and they realize that life from here on will be along an inevitable decline. There is a marked increase in the death rate between the ages of 35 and 40 for employed men, apparently as a result of this so-called "mid-life crisis." The causes of these feelings are often related to questions of technical competence, but much obsolescence is cultural or interpersonal: some older managers cannot cope with the values of younger subordinates; some cannot adjust their sights to radically new organizational goals; and some have become so identified with a faction in the organization that has lost favor over time that they become ineffective.

Others would also go looking for proof of the midlife crisis and find it, as was the case for Gail Sheehy who published Passages: Predictable Crises of Adult Life in 1977.

Thus, the concept of the midlife crisis allowed for it to be used as a plausible - and socially acceptable - reason to make dramatic changes to your life. People were living longer and that also mean you still had time to make changes. If you didn't like your current career path you could still change it. This was particularly important for middle- and upper-class women who could use the midlife crisis as a liberation story: just because you did the "right thing" (i.e., marry a doctor, have a baby) doesn't mean you can't divorce the doctor and pursue a career as a journalist, as Sheehy did.

As society progressed in to the 1980s and onwards the idea of the midlife crisis became more muted. From a modern perspective, there is debate as to if it is a real phenomenon (i.e., is that new sports car recapturing youth, or finally being able to afford one?) and the growing consensus seems to be that an outright midlife crisis is more of a social construct than a real phenomenon. Thus the real staying power of the term is less to do with it being a real phenomenon, but more to do with it being introduced to the public at the right time of social change.

References

Jaques, E. (1965). Death and the mid-life crisis. The International Journal of Psycho-Analysis, 46, 502. Retrieved from https://www.proquest.com/docview/1298193894?&imgSeq=1

Jackson, M. (2020). Life begins at 40: the demographic and cultural roots of the midlife crisis. Notes and Records, 74345–364. http://doi.org/10.1098/rsnr.2020.0008

Footnotes

Life expectancy fluctuated a lot in the early 1900s on the basis of your social, economic, and ethnic background. However, assuming someone survived to 10 years old, generally 50 - 60 would be a reasonable life expectancy.
Anyone that spends time on r/AskHistorians knows that most assumptions about what is normal tend to be misleading though.
Note that the age at which the midlife crisis is expected also creeps up at this point as well, while Jaques was discussing patients in their mid-30s, the idea that a midlife crisis happens in your 40s begins at this time.