at the national level

At the national level, governments can be harder and more draconian than they could be at either the local or planetary level. That’s because they’re dealing with ‘their’ people, but more at arm’s length than the local level. They can afford to be a little harsher with them—for the good of the nation, of course. Thus, while local governments may set the number of children people may have, the national government would determine what penalties citizens would incur for violating these norms. And while such penalties would probably not be criminal in nature, there could be substantial civil penalties in the nature of fines, time spent in community service, prohibition from serving in elective office, etc. The children themselves might be removed to foster care.

It is also at the national level that government has a perfect right to prevent the influx of foreign migrants into the home country. Border checks, visas, even the construction of physical barriers: All these are legitimate methods for controlling immigration—which is to say, controlling population. They are also, conversely, ways of preventing other countries from trying to irresponsibly export their surplus population. Thus they also serve to encourage other countries to control their own populations.