Do pay attention to the title of the post.
Total fertility rate (TFR) decline is a worldwide phenomenon that most likely qualifies as a crisis from any reasonable perspective. You don’t have to be a adherent to “go forth and multiply” (religious or otherwise) to see a declining population as a problem.
It is a threat to economic wellbeing as well as social cohesion and flourishing in general. Fewer people means fewer ideas, fewer relationships, fewer breakthrough achievers, and fewer . . . well, people, a good unto themselves.
I am not speculating that any or all of these are the main cause—just that they may be additional factors.
The dramatic decline in teen (out-of-wedlock) births. Consider teen birth rates of all kinds:
Lyman Stone disputes this as the main cause but does indicate it is responsible for 10-15% of the decline in TFR. As a side note: How much of this decline in teen pregnancy is a result of the rise of the AIDS crisis and its attendant, prolonged fears? It is very hard for a non-Gen X person to understand how dramatic the AIDS concern was for us in the 80s. I guess start by imagining the height of COVID worry extended for a decade plus. Notice how the graph above tracks us as a cohort with about a 5-10-year lag.
Modern technology making extramarital affairs much more difficult. Peggy Guggenheim when asked about her husbands said, “You mean my own, or other people’s?” Although her infidelities did not result in pregnancies of her own, she only had two children, her lovers probably produced offspring in other affairs. Don Draper had it a lot easier in his day than a would-be Don Juan would today. Mobile phones, credit cards, and KYC-in-everything makes a secret second life very difficult. Note that I am not championing this lifestyle.
A decline in divorce rates specifically for younger men who would start new families. Anecdotally both my family tree and my wife’s exhibit a great deal of this—men leaving their wives and children and then marrying and procreating again. The logic would be that childless men do not affect the fertility rate, but men willing and able to father with multiple wives does.
A rise of women (number and status) in the workforce. This one holds up.
Developments in and widespread use of contraceptives (uncomfortable for those who advocate freedom in this area).
Greater access to and a proliferation of pornography (uncomfortable for those who in general advocate freedom in this area and those who specifically advocate/allow for it as a less-bad substitute for out-of-wedlock/teen sex).
I would bet each of these is a meaningful contributor—statistically significant with a negative sign. All but the last two are obviously widely heralded as good/great developments—hence, the “uncomfortable” labeling.
To the extent there is truth in these as explanatory factors, there is a lesson about unintended consequences. Additionally, there is a lesson about low-hanging fruit and the next problem getting harder to solve.