Humans can cognitively evolve. Cognitive evolution can be much faster than physical evolution because it is driven by an environment which includes previous cognitive development.
While our physical characteristics took millions of years to emerge, speaking skills took a relatively short time to develop after humans started to live together in larger groups. Writing skills developed in an even shorter interval when these groups spread out yet stayed in communication.
Both cognitive processes require more than complex brain changes. Each requires per-individual environmental interaction (cultural activities) to make the capacities operational.
Besides speaking and writing, this after birth development of evolved mental abilities includes the abilities (motivation and skill) to infer future conditions which are not repetitions of the past. And the ability to trace backward, from these abstractions, to find what behaviors created them. A skill, many can already perform. A skill given the name feed forward control.
Feed forward control cognitive abilities, create an environment that pushes cognitive development to previously impossible domains of thought. For example, this individual can see, that the future problems we face can not be solved with the mental abilities we have in place. To solve them requires gathering, processing and valuation capabilities we don't have.
For example, if a behavior, is chosen by a process that
balances genetic preferences (human nature) and feelings created by thinking
abilities; that behavior will remain genetically determined until thinking
abilities develop enough that:
Part of these requirements are in place and could be used to change behaviors that have been taking everyone to a destination no one wants. For example, our population being far too big for the desires of its individuals (resulting in ever increasing conflict) is an inferential conclusion within existing cognitive abilities.
Procreative behavior can be backed out from a causal model as an important contributor of this conflict.
The one child per family behavior, can be tested in simulation as an alternative behavior and can be shown to produce a better future condition.
The ability (universal among the globe's constituencies) to value that future over the future of present behavior is not developed enough to cause the selection of the one child per family behavior.
Thus we might conclude that the one child per family behavior remains not chosen because humans have not yet evolved their cognitive abilities adequately to make the change in behavior possible.
That should be enough to push for the next small step in cognitive evolution to ensure our graceful survival.