I don't think any simple trick like that is enough. If people are allowed to choose their own paths, they will make different choices, and every aspect of the society has an impact on the choices they make. If we want a society where people have the right number of children on the average (not too few and not too many, as in the societies discussed in the blog), everything must be designed around that.
For example, if having kids is optional, many people will choose to not have kids. Then those who do have need more than two kids on the average to sustain the population. But we have this cultural ideal of a family with two kids, which impacts the design of everything from homes to cars and from furniture to hotels. As well as the expectations on how much effort parents should devote to support each child.
For example, if having kids is optional, many people will choose to not have kids. Then those who do have need more than two kids on the average to sustain the population. But we have this cultural ideal of a family with two kids, which impacts the design of everything from homes to cars and from furniture to hotels. As well as the expectations on how much effort parents should devote to support each child.