A_Gupta wrote:Let's start with 100 women all of the same age.....
Your example assumes beginning with a zero base. That is not the case.
Not sure what you mean. In any case it is meant to be a simple example to illustrate that the population growth does not stabilize immediately upon achievement of replacement TFR.
Secondly, you assume TFR at the replacement level not exceeding 2.0-2.1 from the beginning.
Please read the example again. I'm counting the female sex only, to simplify bookkeeping. My initial TFR is 2 girl children per woman, which would translate to a little over 4 children per woman in the real world.
That is, my example uses an abrupt drop from twice
the replacement rate to the replacement rate, and shows how it still takes some time for the population to stabilize. The real world typically, and in India most certainly, does not show such an abrupt drop and so the time it takes for the population to stabilize is even more.
In India's case the TFR has been above the replacement rate for generations and is now trending now. What that means is that the incremental annual inflow into the child bearing cohort now should be lower than the incremental annual outflow from the child bearing cohort.
No. The incremental annual inflow into the child bearing cohort is very high because of the youth bulge. E.g., in 2014, the women in the 40-44 year age group soon to leave the child bearing cohort are about 40 million; the women in the 15-19 age group number around 55 million, and the younger age groups are similarly large. My example was constructed to show a one-generation youth bulge.
Assuming constant life expectancy the population should therefore decrease after 2020 if GOI TFR rates are to be believed. But nobody is forecasting any plateau in India's population till 2050-2060. So either the population numbers are wrong or the TFRs are wrong. Or Indians have suddenly got an extra long life expectancy.
No. Either do the math, or demonstrate with a model. Or perhaps sharpen your understanding of what TFR means. I showed you this in an example of an abrupt TFR drop from twice the replacement rate down to the replacement rate, and "momentum" carried the population to increase some more before stabilizing.
Look, there is no doubt that the TFR is trending down, what is not clear is the rate at which it is decreasing and that unfortunately has big implications for the size of the overall population. If India's current population was 130 million it would not matter much if the plateau was reached in 2050 or 2100. But when the current population is already ~1300 million, a few decimal points can make a difference of hundreds of millions 40-50 years out and that has implications for food, water and energy resources as well as jobs.
Yes! And conversely, the TFR is falling faster than the UN population projections (which do all the math needed, including the life expectancy changes, the continuous rather than abrupt drop in TFR, etc., etc. and which are updated on some schedule as new data comes in.) and therefore we already have an upper bound for what the population is going to be in 2020 and 2050 and so on.
In the "medium" variant of the UN's population projection for India, India's population peaks at around 1.754 billion around 2068, and is around 1.659 billion at 2100. But the "medium" variant has India's TFR at 2015-2020: 2.32. The "low" variant has India's TFR 2015-2020: 2.09, and in the "low" variant, India's population peaks at 1.517 billion in 2045 and is 1.494 billion in 2100.
Notice, the "low" variant has India at replacement TFR in 2015-2020 and population peaks in 2045.
The other point is that at present, India's demographics is going to somewhere between the UN's "low" and "medium" variants; and if the TFR continues to fall faster than the UN's "low" variant anticipates, then the future will be better. I'll put it this way - during 2000-2010 it looked like India was following the UN's "medium" variant projection; and now it appears India may be approaching the UN's "low" variant. And the downward momentum may be such that India may beat the UN's "low" variant.