> Why is this true? Because of something called the "Dependency Ratio": http://en.wikipedia.org/wiki/Dependency_ratio http://en.wikipedia.org/wiki/Dependency_r

> Why is this true? Because of something called the "Dependency Ratio": http://en.wikipedia.org/wiki/Dependency_ratio http://en.wikipedia.org/wiki/Dependency_ratio The basic concept is that in order for there to be a high quality of life for most people in a society, the majority of them need to be capable of doing economically productive work. Otherwise you have a large number of dependents (juveniles and/or retirees) whose sustenance must somehow be garnered from a relatively small number of workers. In China, for example, demographers talk about the "4:2:1 generation", which is the long-term effect of improving longevity combined with the one-child policy. This means that there could be four retired grandparents and two retired parents to every one working individual. Somehow, the social services for six people would have to be supported off the tax revenue from one. That's just not possible, and could very easily lead to a bad outcome for society as a whole. (This isn't to discount the problems that come from overpopulation -- I'm just pointing out that it cuts both ways). Too-rapid population growth can also lead to high dependency ratios -- in the form of children rather than the elderly -- and this also makes it difficult/impossible to support adequate social programmes. Historically, the best-performing economies tend to be those with very low dependency ratios, which is typically the result of a baby boom that is quickly followed by a baby bust. That produces a bulge of economically productive workers, the revenue from which can be spent on things like infrastructure and investment rather than social-support programmes which have less of an impact on GDP. (Incidentally, this is a large part of the reason why China is going so bonkers with infrastructure development, building high-speed rail and ghost cities for which there is arguably no market. They're doing this because now is the only time they can. In 15 years, China as a whole will be substantially wealthier -- with enough people who are genuinely able to afford high-end condominiums and high-speed rail -- but at that point their youth bulge will be gone, scarcity of labour will drive up wages, most government revenue will have to go to healthcare and social services, and they'll have no ability build that kind of infrastructure. So they're building it while they can, well in advance of their actual need for it, which is actually pretty damn clever.)