There’s some interesting reading about demographics and Global birthrates: as you might have learned from the news, the birthrates in meny advanced (mostly western) countries are declining, in some places to such low levels that should the trend continue, those populations may become extinct within a handful of generations.
While in other parts of the world, fundamentally the non-developed world, birth rates are – and have been for a long time – alarmingly high.
It appears that most experts on demographics agree that in order for any population to sustain itself, the birth rate, that is, number of kids per woman, should on average be 2.1.
In many western countries, birth rates are now well below 2, which means that there are fewer children born than there are people dying, e.g per year. Which in turn results in a diminishing population.
Anyways: I wrote a demographic simulation that allows me vary a number of parameters, most fundamentally the birth rate, but also other params such as initial population size, number of generations for the simulation, expected lifetime and its distribution etc.
Below some images from some simple simulation runs.
It is indeed very clear that a birthrate of about 2.1 will result in a balanced demography – anything less than that and there is a clear risk of extinction; anything over and above 2.1 will result in an explosion of population. The larger the birth rate, the faster the explosion.
In all the images below, the initial population is 10.000, and the simulation runs for 10 generations, which clearly is not enough to fully see the dramatic change near the inflection point of birth rate 2.1, but you should at least be able to observe how very sensitive to birth rates polulation growth is.
Looking at the first graph, where birth rate is 3.5, which is a not uncommon birth rate in non-developed societies, you will notice that in 10 generations, the population will grow from 10.000 to 4.5M! Now, 10 generations, say that corresponds to some 250 years, might be too long a time for causing any immediate concern, so it might be interesting to note that with the same parameters, the population will grow by factor almost 6 in just 2 generations, and by factor almost 17 in just 4 generations!
Similarly, at he other end of the birth rate spectrum, populations with birth rate lower than 2 should feel reason for worry – if such a low birth rate stays for a handful of generations, the entire population might collapse.