Japan and Korea seem like bad comparisons, since they're relatively rich. It's fairly well known (read: I don't feel like looking it up right now) that there's a strong negative correlation between wealth and birth rates. But I don't know what the right comparison would be (China is kind of incomparable) nor what the answer would be.

Perhaps the best comparison would be India, and the disparity in birth rates there is stark, but there are massive other differences as well.