The three necessary and sufficient ingredients for evolution are: 1) Inheritance, 2) Variation, and 3) Selection.
Let’s consider fertility using this lens:
1) Fertility rate is heritable, 2) Fertility rates vary from person to person, and 3) People with higher fertility have higher reproductive success (it’s almost a tautology).
That struggles to explain how fertility rates have ever dropped. For higher fertility you would need stronger biologic instincts, the present lack of which suggests they would conflict with something else important for viability, a high "materialist profit" from having having children, which the man's child protection services will catch on to if your group grows big enough, or a "sense of duty", which again seems increasingly hard to maintain as the group grows for an array of reasons.
> That struggles to explain how fertility rates have ever dropped.
No, it doesn’t. The environment we inhabit has changed drastically in the last 150 years. We were well adapted to the environment we lived in 150 years ago. We are poorly adapted to the environment which we live in now. That will change as evolution does its thing.
For a glance into the future, take a look at the habits and lifestyles of people with lots of children. There is an interesting bi-mods distribution, btw.
Let’s consider fertility using this lens:
1) Fertility rate is heritable, 2) Fertility rates vary from person to person, and 3) People with higher fertility have higher reproductive success (it’s almost a tautology).
I’ll let you work out the implications.