Could it be economic?
Discussion
Only marginally imo
History shows that birth rates plunge in periods of economic hardship.
Hardship, or inequality?
I know it's not easy to distinguish this two thing, but I think that inequality means that poor people generally have less control over life choices
If everyone's equally poor, my hunch is that we'd see more children
They’re both relative to the society and era.
When there was more absolute poverty prior to the industrial revolution, families were much bigger, especially for wealthier families. However, even poorer families had many more children but only about half reached adulthood. Children provided labour in agricultural societies and also took care of elderly parents.
Barring specific confirmatory evidence, fear and self-absorption as you put it are just as likely to be effects as causes of low birth rates.