In cases of overpopulation, wars and disease have sadly been “efficient” ways to reduce excess population growth in the past, and paradoxically many of the most acute population problems have been in regions of relative peace, albeit usually an enforced dictatorial peace. In such regions, natural migration to reduce overpopulation is also restricted.
Today’s largest population growth is among Muslims, and this is by Islamic design and management, following a philosophy/religion of ultimate conquest through demographics, and through dhimmitude enslavement for non-believers. This is unlike the second-fastest-growth population of believers, the Hindus, which does not purport to be a conquering religion. However, the two are of course diametrically at odds in the India-Pakistan flashpoints as well as in Malaysia. Christianity in some respects is still a rapidly-growing religion, but it is less population-based or racial, the new Christian growth is largely within some of the largest populations in the world, competing with Islam and Hindus from within and without.