Re: Only one number that really matters
The following might be considered counterintuitive, so I'll type slowly.
High child mortality leads to overpopulation. Read it again.
Most humans instinctively want to reproduce themselves. They want high odds of success in this mission. 50/50 isn't good enough. 95% is probably about right. 100% is not required.
If the child mortality is low and you want two kids, then you have two kids. This will almost certainly lead to several grandchildren and one would eventually die happy.
If the child mortality is high and you want two kids, then you might need eight babies to provide 95% odds of having two survive. But under these circumstances the average could be four that survive. In other words, to guaranty two the average would overshoot and result in overpopulation.
As soon as the child mortality problem is fixed, population growth tapers off.
I'm convinced that this is the root cause. It may then be layered over with cultural and religious reinforcements. But if you fix child mortality then the human overpopulation problem will fix itself in a generation or two.