Comment Children used to be super cheap labor ... (Score 1) 75
... and basically "just happen" as a side-effect of normal life. You'd have a surplus even with half of them dying off in early childhood. Today they are ultra-expensive pets that can easily cost north of 100 000 Euros for families with demands and expectations in developed countries. And selection isn't brutal anymore, it's basically non-existent with modern medicine. Urbanisation, electric power and lighting, education and other effects have turned producing children into an exception rather than the rule.
An additional effect I'd call it the decadence/affluence effect. Don't know if there is a term for this, although the effect has been recorded in history. The late Roman Empire being a prominent example, but there are other sunken civilizations that went through similar phases before vanishing swiftly.
Instincts that drive mating are fairly low-level and are elevated by positive stress and porking your sweatheart being the primary highlight of your daily life. Overtune those with other distractions and a non-scarcity environment, modern contraception and virtual sex and people have less reason to engage in mating which becomes more and more complicated as standards rise into the absurd. The result also being less mating and birthing. See Nigeria vs. the rest of the world today for details.
This is also the prime reason why revelation cults that have "pushing out babies" as a basic duty actually have an evolutionary advantage and anti-theists are prone to dying out once they emerge in high cultures. Being an anti-theist myself I've recently been discovering more and more distinct advantages of adhering to the mind-virus of an abrahamic revelation cult such as Christianity or Islam. The cultists do and will survive the decline of high civilization(s), that's for sure proven IMHO.