I first heard about the population explosion when I was a stripling of 15. Our Year 9 geography teacher told us that global population would reach 8.5 billion by the year 2000. Back then, in 1960, it was less than three billion.
Well here we are in 2012 and global population stands at around 7 billion. We're unlikely to reach 8.5 billion before 2030 at the earliest. The population has exploded but not nearly as fast as was predicted.
So what happened?
Population growth rates in any one country depend primarily on four factors:
(1) Proportion of women of child bearing age
(2) Average lifetime number of babies per woman. This number is called the total fertility rate and is abbreviated to TFR
(3) Net migration
(4) Death rates
For the Earth as a whole we can probably forget the third factor. So far as we know there is no net migration to Earth from Mars, Alpha Centauri or anywhere else.
Longevity is increasing in most countries so age-adjusted death rates are all falling.
The really important number is TFR. A TFR of 2.1 under modern conditions is usually considered replacement level. With a TFR of 2.1 each cohort of women will just replace itself.
It was not always thus. In mid-eighteenth century London, for example, replacement level was somewhere between three and four. Child mortality rates were much higher then.
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