And we aren't likely to, unless they arrive from other countries. The direction of things is now the same in developing countries as they grow wealthier. The fertility rate in India today is lower than the American rate in the 1950s. Fertility rates in most sub-Saharan African nations are falling steadily. Wealth, in short, seems to be very effective in curtailing fecundity.
Nobody knows exactly why, but the correlation is much too strong to question.
The rich secure their genetic posterity through quality, not quantity. Wealth, then, is the best answer to the sprawl of the human genome. Green is what people become when they feel personally secure, when their own appetites have been satisfied, when they do not fear for the future or for their own survival or their children's. It is wealth that gives ordinary families the confidence to be generous to the world beyond. It is the rich who can afford poverty, so to speak.