I think that would only be true if the dominant paradigm in the society were that of capitalism. There are other paradigms that have hardly ever been explored.A world were only intelligent people were bred would be a very horrible world in the long run. Imagine a world where everyone was inclined to go to college to learn higher end jobs, and had the potential for better social understanding. With everyone wanting the better jobs, who would be the farmers, the mechanics, or the department store cashiers? Intelligent people wouldn't want these jobs, yet they still need done. Now imagine if the government came up with a system to fill these jobs, whether it be by lottery, chosen by birth, or any other means. The person chosen would not be happy with their life.
As to why people in poverty pop out babies - as I said in the other thread, it's because the social cost of having children for people living in poverty is negligible, and in fact it's actually advantageous for people living in Third World conditions to have children. In an agricultural or sweatshop society, children manage to sustain themselves around the age of eight. In a post-industrial society, where you have to send kids to college and they generally don't even work for a living until they're out of graduate school, having children becomes a much larger social cost, and so people abstain from doing it nearly as much. So, in short, one solution to overpopulation would be to stop hoarding so much for ourselves.
That said, I think the dangers of overpopulation in the Third World to the planet as a whole are frequently overstated. Certainly, if we continue to be as generous towards the Third World as we currently are, the number of people starving (it's currently 30,000 per day, and growing) will only continue to increase. However, the people living in the Third World are not the ones using all the resources. The USA contains 4% of the world's population, and consumes 35% of the world's resources, while the rest of the world, containing 96% of the world's population, consumes 65% of the world's resources. That means that, per capita, Americans use over twelve times as much resources as the rest of the world, averaged out. When you compare Americans to the poorest thirty percent of the world, it's even larger - I think it's a hundred times per capita or something.