I have no proof and no source whatsoever BUT! Nature surely has a way to combat overpopulation. Animals stop reproducing when their environment stops supporting life properly. Humans aren’t much different. Too much stress, either from being sourrounded by too many people, or from our environment not meeting our demands, make us not want to have children. We also don’t NEED children anymore, since society will take care of us in place of children. With a constant pressure from society to always work more, harder and better, it’s no wonder we don’t want children. Our environment doesn’t support us anymore, we lost reason to procreate, and so nature corrects by not making us do so.