Tuesday, July 21, 2009

Has the world reached replacement rate fertility?

At first glance, the question might seem odd, given that the global fertility rate is currently estimated by the UN to be somewhere around 2.5, well above 2.1, which is usually cited as the replacement rate. However, 2.1 is a number that is often bandied about somewhat lazily, even by people who should know better. It is important to remember that the replacement rate is not a constant and the 2.1 number is only really relevant for developed countries where the actual replacement rate is usually in the 2.06-2.08 range.

The replacement rate depends on several factors: age-specific fertility rates, female mortality rates and sex ratio at birth. Indeed, a useful approximation for the replacement rate is simply the inverse of female chance of survival till the mean age of motherhood times the proportion of female babies at birth. To give an example, in Norway the mother's average age at birth is 30.3, 99.2% of women survive till that age and 49% of births are girls. Thus, the replacement rate would be 1/(0.992*0.49)=2.06.

This is an approximation, the actual calculation of replacement rate is fairly complicated and involves using things like life tables, age-specific fertility rates etc. and requires a level of data accuracy that isn't available in most countries. The British Office for National Statistics has a useful explanation here for the technically inclined. The approximation method above is much simpler and about as accurate.

Now, why is this important? Well, because replacement rates varies widely. Norway in the example above is at the lower bound of the variation but at the upper bound you have countries like Sierra Leone where it is around 3.2. Espenshade et al. published a paper in 2003 (behind firewall) where they tried to calculate replacement rates in different countries. Espenshade also co-authored a more recent open-access article on this subject on populationaction.org. Key quote: "Four fifths of the world's 6.5 billion people live in countries in which replacement rate fertility is above 2.1 and 215 million people live in countries with replacement rate fertility higher than three children."

When countries like India target a fertility rate of 2.1, it is important to remember that this is actually well below replacement rate there. Indeed, for the world as a whole, replacement rate is slightly above 2.3 children. That means we're not quite there yet, but almost. The point is that when we'll get there depends both on the evolution of the fertility rate and of the replacement rate. The latter will again depend on a number of things: mortality rates, the gender imbalance at birth in Asia and also the increasing proportion of births that occur in countries with high replacement rates.

Update: A commentator asked where one could find replacement rates by country. I have been able to find an open-access source here.


Anonymous said...

Fascinating. Any idea where one could find replacement TFR (RTFR?)calculated for all countries?

Interesting too that - according to the article you linked - skewed gender ratios don't affect RTFR that much - not having worked out the math, that surprised me.

I would think it quite anomalous to find countries with elevated RTFR's and TFR's below those RTFR's - but looking around, Bangladesh must be close (TFR 2.4-2.8, depending on the source), and India too (2.7).

(Incidentally, do I remember correctly that the gender imbalance is worse in India/South Asia's Hindus than its Muslims?)

Southern Africa would be another example, presumably because of its combination of relatively high development levels and tragic HIV prevalence. But apart from that, doesn't it seem that South Asia's combination of lowish fertility with highish mortality is really quite remarkable?

Aslak said...

I updated my post to show where you can find replacement rates. You're right that South Africa and Bangladesh are probably both below replacement rate.

I don't know if there is a religious difference when it comes to the gender imbalance. I do know that there is a regional one though, with the Northwest being worse affected than the South, if I remember correctly.

Anyway, I'm sure that if you go through the numbers you can find more countries in the same situation as Bangladesh and South Africa. Swaziland is one example- My guess is that it will be true for an increasing number of countries.

Anonymous said...

Interesting topic. That source with the RTFRs is also very interesting. I have been looking for a source such as that for awhile.

Armenia and Azerbaijan might encounter some fairly harsh demographic troubles even compared to Russia or perhaps Bulgaria. Looks like Iran could be in the same submarine too.

I wonder how possible it is to get an accurate estimate for North Korea? The replacement TFR in the source was a little bit below the world average. Regardless, it is likely that North Korea has sub-replacement fertility though. If mass emigration from NK becomes a reality in the future than its population decline would completely eclipse the other states in the region (rather severe though their declines will probably be).

Aslak said...

The short answer when it comes to North Korea is not at all. There's just no way of getting reliable statistics from certain countries, either for political reasons like in North Korea or simply because conditions on the ground are too difficult like in Somalia.

Damien Sullivan said...

Enlightening on replacement not being a constant, and I'm glad the linked article gave that table. Seems to me that the number is more useful for its own sake than an indicator of other things better indicated by the factors going into the calculation. Of course looking at female survival and sex ratio are more powerful than diluting them with each other. And seems obvious to me that survival has a higher range and thus more influence, though admittedly it's more obvious in your version of the formula.