Derivation of actions to reduce population to sustainable and maintain it there
Population size is dominated by the number of births each year. How many babies have to be born annually to make a stable 50 million person population? If we assume that the average age at death increases this century to 100. And we assume the destination population has 100 equal size age cohorts. Then there has to be 500,000 in each age cohort and that means 500,000 births each year.
Population at the end of 100 years is the sum of the births for 100 years, minus the sum of the deaths for 100 years. The death count is all of the 7.5 billion who live today because they all die of old age. The sum of 500,000 births each year for 100 years is 50 million.
So what behavior creates 500,000 births per year. Initially, It cannot be the collective result of personal decisions driven by genetic, culture, or economic forces. Parents need a little outside assistance. For example, a parent's decision has to be guided by a civil law that limits the total number of births to 500,000. For example, a lottery that gives out only that many birth permits. Without a permit you are not allowed to have a birth. So the behavior that achieves the 50 million person population is a vote for a birth lottery law.
In this case a parent's "no-vote" for such a law is like the slave owner's no vote for abolishing slavery. The votes "for the law" come from people who can see the benefits of the law are bigger than the injuries imposed by the law.
As shown here the benefits for voting for such a law are not having civilization collapse, a 95% die off of the human population, a destruction of the environment, the survivors living at subsistence with little chance to rebuild society.
The injuries caused by the law are significant but much smaller. Let me outline them. The law's injuries will be most intense for 50 years after which they will be almost non existent.
Assume that globally there are 140 million women who give birth each year. About half of those births are planned. For the unplanned half, these mothers would not be a largely disappointed if they did not have a child. So that leaves 70 million who annually want a child facing a law which allows only 500,000 births. The injury is 139 of every 140 women to go childless.
While this is an intensely hurtful period for these parents, these injuries, from civilization's point of view, lasts for only 50 years. I will explain why, but first consider that it took humankind 3000 years to create our overshoot predicament and this predicament has to be resolved this century before overshoot ruins everything.
Injury resulting from the law stops in the 50th year because that is the menopausal age. In the 50th year of the lottery the fertile women cohort is the same size at it will be for a population of 50 million. Even thought it will take 50 more years before the total population will be 50 million.
For a population of 50 million, (or a population when the fertile woman cohort is the same size as a 50 million person population) 500,000 annual birth permits means enough for each woman to have 2 children during her lifetime. The reason I say that few woman are injured by the law after the 50th year, is that about 1/3 of the women will have no child by choice or health so their two permits are released and other women can use them. About 1/3 of the women will have one child by choice or health and one of their permits will also be released. That leaves the women in the third group with their 2 permits plus 3 from the women that don't want them. The third 1/3 of the women, if it is important to maintain the population at 50 million, must have an average of 5 children. If a woman wants only 4 children, another can choose 6. If a woman wants 3 children another must have 7. For every woman that has two, another must have 8. So one can see the law which limits the community to 500,000 birth permits is not producing very many injuries after the first 50 years.
Thus I conclude that the behavior that must be taken is a vote for a law that limits annual births to 500,000 because it produces less injury to humankind than not having the law.
What happens if in any one year, women collectively do not want to have 500,000 births and some permits go unused. This is as serious a problem as too many children. Unaddressed, the population will decrease until it goes extinct. More immediately all the seats in the symphony or some other human endeavor will go unfilled. In such a small population each person is important. So maintaining the population is an important goal. As part of the new social contract, there will have to be an incentive to use unused birth permits." The whole community has this collective need. The whole community collectively has to fund these incentives.
5/20/2015 revised 11/4/2016