World Population imbalance between the sexes

[Jester]

A combination of local laws, a strong propaganda campaign, and tying foreign aid to reasonable population control could make the situation go the other way, and a population of 4 to 5 billion by mid century is not impossible.

Not impossible, but only in the case of a complete stop on all reproduction. We have 40 years until the mid-21st century. We have 6.5 billion people on the planet. Mortality is about 0.85% each year, or about 55 million deaths. Assuming zero reproduction, and holding deaths constant*, it would take around 30 years to get population down to 5 billion, and almost 50 to get it to 4 billion. (It would drop like a rock after that, because we'd lose our whole workforce to "retirement" - not that there would be retirement in such a world.) Although maybe that's what you meant by "reasonable population control."

If we assume even a tiny trickle of children, we don't come even close by mid-century. Even 1/5 of our current birth rate would be enough to cut population decline in half, extending our timeline out to 2070 or later to even hit 5 billion.

My bet is, we overshoot to about 10 billion, then crawl back down slowly to 8 or so. Most of India will get over the demographic hump (...) soon enough, and Africa and other parts of South Asia will follow suit in 20-30 years.

-Jester

* There would be an incresing resource surplus from having to raise no new children for the first 18 years, followed by a slow shortage as we ran out of kids to enter the labour force, and lost productive capacity. Also, without new children we would have to subtract infant mortality and deaths in childbirth, which should lower the mortality rate.