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A declining fertility rate doesn't just change change the population, but also the relative distribution of each age group. A fertility rate of "N" means the next generation will be "N/2" times as large as the one before it. Because of this you can easily model what a society, composed of successive generations of a fixed fertility rate, would look like. To keep things simple, imagine everybody gives birth to all their kids at 20. When they actually do doesn't matter, it would always converge on the same ratios: ---- Fertility Rate of 2 (average age = 40): 100 80-year-olds, 100 60-year-olds, 100 40-year-olds, 100 20-year-olds, 100 newborns. Fertility Rate of 1 (average age = 63): 100 80-year-olds, 50 60-year-olds, 25 40-year-olds, 12 20-year-olds, 6 newborns Fertility rate of 0.5 (average age = 74): 100 80-year-olds, 25 60-year-olds, 6 40-year-olds, 1 20-year-olds, 0 newborns ---- You can also estimate the ongoing population rate of change for each fertility group by removing the elderly generation, adding a newborn generation, and contrasting the new population sizes: ---- Fertility Rate of 2: 500->500 = 0% population change per 20 years Fertility Rate of 1: 193->96 = 50% population decline per 20 years Fertility Rate of 0.5: 132->32 = 76% population decline per 20 years ---- In the developed world most fertility rates plummeted around 1970. So, assuming nothing radically changes, we should start hitting our 'equilibrium point' about one life expectancy away, so sometime around 2050. |