“The figures show that immigration numbers approximately the same and for the last few years exceed the births…”

Exactly!

Immigration was for a long time about the same as the natural population growth rate, that is; births in excess of deaths. Lately it has pulled away from it, as it has increased more than the birthrate, despite the increase in the birthrate spurred by the baby bonus.

So our population growth was for many years composed of about 50% immigration and 50% domestic births. Now it is more like 70/30.

How could that possibly be if we had a national birthrate that was below replacement level?? How could there be any component of our population growth due to births if the birthrate is below replacement level?

The Australian Bureau of Statistics (as for 24/9/07) reported that there were 273,500 births and 135,400 deaths.

So we are still chugging along with population increase from our own Aussie efforts apart from any outside help.

That will continue to be the case until the present bundle of females at breeding age get beyond it - something like a generation to go yet.

Net overseas migration (arrivals less departures) at that date provided about another 162,600 people.

Ah, it all adds up to being enough to bring joy to the heart of Peter Costello and similar antediluvian troglodytes unwilling to come out from their caverns and into the glaring light of the real world.

The major reason that the population is growing is that the life expectancy is increasing. Australians are not replacing themselves but rather living longer. The problem is not with the present batch of females, but with the present batch of over 65s who are inconsiderate enough not to croak.

The average births per women of 2.1 is a zero sum game. If the number is lower than this the population will eventually shrivel, increasing life span will only temporarily distort the figures.

Democritus

The following is from Dr David Kault, President of Sustainable Population Australia, North Queensland Branch;

Fertility as measured by TFR (total fertility rate) dropped below replacement in Australia for the first time in modern times in 1976.

Replacement TFR, given the slight imbalance between male and female births and the slight mortality before reproductive life is slightly above 2 and is regarded as about 2.075. Australia is now at 1.81, though a couple of years ago, before the baby bonus it had declined to 1.72. Therefore according to this measure, we are about 15% per generation below replacement and if we take this TFR at face value this would cause a halving of the population over about 150 years in the absence of migration.

However, there are a couple of important factors which mean that a below-replacement TFR may not correspond at all to a population that is declining in the absence of migration. Indeed, at the moment, even without migration, Australia's population is rapidly increasing as shown by the Australian Bureau of Statistic's population clock site: http://tinyurl.com/27ubgr.

The population growth can be crudely summarised by stating that every 4 minutes in Australia there are 2 births, 1 death and 1 immigrant (actually now closer to 1.5 immigrants).

We have a situation with twice the number of births as deaths despite below-replacement TFR.

This excess of births over deaths reflects the fact that there is a relative excess of people in Australia in the age range 20-60. Those in their 20's and 30's are in the middle of reproducing, but they are not counterbalanced by a proportionately large population in old age who will be dying. To explain this another way - consider a population made up solely of 20 year olds who all wanted just one-child families - those families to be created by the time they were 30. TFR would then be 1.0, well below replacement of 2.07, but because almost no one would die, in 10 years the population would have increased by 50%. This effect, with births greater than deaths despite below-replacement TFR,

continued

is expected to persist for about 30 years so that births are not predicted to fall below deaths until the 2040's.

Sometimes the effect here can be referred to as the first and second echoes of the post WWII baby boom (the creation of the grandchildren and later the great grandchildren of the large numbers born in the 1950's - and these numbers will be large because of the size of the 1950's generation).

There is another lesser known effect. The TFR is a cross-sectional measure - adding up the number of babies had by 15-20 yr olds, 20-25 yr olds, 25-30 yr olds, etc..., in any one year. It will be an accurate reflection of the number of babies women will have in their lifetime if today's 15-20 yr olds have exactly the same number of babies in 30 yrs time (when they will be in the 45-50 group) as today's 45-50 yr olds.

If the average age at reproduction is rising, as in Australia at 15%-16% per year, the TFR underestimates number of babies per woman by 15-16%. With a correction for this effect, Australia may well not be at under replacement fertility.

To explain the effect, consider a society in which all the women decide to postpone having babies for one year. At the end of that year the measured TFR would be zero, but if the babies are only postponed but not forgone, the number of children to be born to each woman would be unchanged. Now say the postponement was for half a year - but with catch-up for the postponed babies being not at the end of the half year, but by extending the reproductive period by half a year, then the TFR would capture only half the number of babies that women would be having in their lifetimes and so TFR would underestimate reproduction by 50%.

With maternal age at childbirth increasing by 15% per year in Australia over the last 23 years or so, this effect indicates that the TFR is underestimating babies per woman by about 15%.

As the effect of postponing births would deflate the birth / child figures in one period but inflate them later, this is once again only a temporary distorting factor.

I also note that in the quote Dr Kault was only giving it as an example of possible causes of errors in the calculation of the birth rate and not proposing it as reality for the following reasons:

As women have been getting involved in careers since the 60s and the birth / woman has been below 2 since 1976, the tail end of this effect would have been apparent by now unless the suggestion is that suddenly in the last few years some women have taken advantage of the baby bonus whilst even more have decided to postpone pregnancy even further.

Combined with the correlation between later births and smaller families, I would think that this would be an extremely tenuous proposition.

I would concur with the conclusion that he made previously that the population without migration would decrease. The distortions he mentioned would only change the amount slightly.