Those nations with high rates (>5 children per women let us say) tend to be economic basket cases. Now my thinking is that one is causing the other - but it is the bleak economic and mortality prospects that cause the high birth rate. Children being the retirement plan of the third world.
Those nations with lower than replacement rates tend to be moribund Socialist nanny States. They are now trying to incentivise reproduction and having some success with it, because lo and behold - people respond to economic incentives!
When there is an societal, economic or other incentive to have more children - birth rates will increase - it really isn't much of a problem.
Even according to the scenario you spell out - human population is expanding and will do so for the next forty years - then it might contract - but only if a recent trend hold exactly the same for the next forty years!
See post #155.
In comparison to developed countries, agrarian societies today are economic basket cases. But in this which-came-first-the-chicken-or-the-egg scenario, it wasn't the high fertility rates that caused better or worse economic situations.
Societies that modernized and "succeeded" transitioned from the universal norm high fertility rates to developed world lower lower fertility rates.
Societies that failed to develop into first world economies did not transition from the universal norm high fertility rates to developed world lower fertility rates, they simply maintained the prior universal norm.
High fertility did not cause modernization. Neither did it cause economic stagnation.
Economic development and "success" in first world nations lead to lower fertility rates.
As third world nations transition to first world status, all trends indicate they too will have lower fertility rates.
Economic prosperity leads to lower fertility.
High fertility does not cause economic stagnation, or else America would never have become an economic world leader.
And as Julian Simon pointed out, population growth is not an impediment to economic growth. In fact, it may be a necessity for it.
But population contraction has always been associated with economic contraction.