An ongoing question I've talked about a few times before is that at least some of our toughest social problems are really technology problems. That is, not problems of a lack of technology, but of a surfeit of neutral or even beneficial technology which is having unexpected negative side effects. Mass illegal immigration is mostly a problem of cheap transport, for instance.
I mostly think about the declining birthrates in much the same way as I think about the increase in obesity (which deserves its own post for sure). Specifically, that technology has produced an environment so unlike that to which we’re evolutionarily adapted that people’s instincts no longer produce reliably good outcomes.
In other words, reliable contraception and abortion has been like a basilisk. It short circuits what had previously been a very successful evolutionary adaption which used to have high reproductive fitness. It leaves humans like the moth circling the light bulb, thinking it is the moon and flying in circles until it drops of exhaustion.
What are the instincts that people have with respect to children and reproduction?
1. People have a very strong, uncomplicated and concrete desire to have sex, ideally right now. They don't need marketing campaigns to make them want to do this. Oddities like Japan aside, people seem to have no problem getting laid.
2. People have a somewhat strong, but quite complicated, abstract and malleable, desire to have children, at some point in time.
3. People have a very strong, uncomplicated desire to love and care for the children they have.
#2 and #3 are deliberately split into two parts. As the great Gary Becker put it, you don't love your children as much as you learn to love your children once they arrive. There was usually the option to have one more child, which you chose not to do, often as part of a quite sensible cost/benefit tradeoff. Of course, if people had an unplanned pregnancy and had another child anyway, they'd still love and care for the child. But the fact that each child gets loved intensely once it arrives doesn't cause people to want as many children as possible. The love only kicks in after they arrive, and the prospect of loving another child in the abstract does not exert nearly the same overwhelming pull.
In other words, traditional reproduction worked primarily through #1 and #3. A strong desire to have sex ensures children are produced with fairly high regularity, because birth control is either non-existent or unreliable. A strong desire to care for children once they arrive ensures they live to adulthood if resources allow. #2 served mostly as a general background reinforcement. This is the environment we all lived in from 10 million BC until the 1950's or so.
The whole idea of it being a contentious question whether you chose to have kids or not is, as far as I can tell, a shockingly recent question. If the only way you could so choose would be to either a) not get laid, or b) rely on methods that require practice, discipline in the heat of coital moment, forward planning and/or health risks, the discussion would be largely moot.
With birth control, childbirth has been largely disconnected from being a necessary consequence of getting laid. It seems that most unplanned pregnancies are teenagers who are still learning the ropes of birth control, the very poor who simply can't afford it, or people with very low forward planning skills. But regardless of how you cut it, in the modern era it is very easy to take precautions that mean you can have sex for an extended period and not get pregnant.
As a result, we’re now expecting the second, weaker desire to do the job where previously the heavy lifting was done by the first. You have to choose to have children. Is it a wonder that this doesn’t wholly succeed?
This doesn't mean that the problem is impossible - social pressure can be a powerful force, if the right motivations and incentives are set up. But make no mistake, we're expecting new social engineering to reproduce a result that was previously done just by our evolutionary adaptions.
I suspect that we are only recently finding out that the majority of human survival and reproduction was actually driven by unplanned or unwanted pregnancies. We now suffer from a want of unwanted pregnancies, and we don't know how to make up the difference.