Conflict seems to be the natural end product of human activity. Israelis and Palestinians could learn this from the last remaining wall of King Davids temple. The wall, first built more than millennium ago, is a catalog of groups that took Jerusalem by force and then were driven out. Davids builders laid the bottom stones still visible today.. A second group, conquered his kingdom, knocked down his wall, and built a new wall on his bottom courses. This second group was driven out. Their wall was knocked down. And the third group rebuilt again. The tapestry of stone courses records no less than five civilizations.
If the present group is not different from its predecessors, it should expect to be driven out. A different future requires a different behavior. However, this behavior is so hidden few previous groups ever thought to take it. If they could conceive of the behaviors benefits, they found the price too high to pay.
The behavior requires every member of the group and each member of each surrounding group (now six billion people) to parent only one child.
The resulting rapid population decline (not a lower population) and the expectation of successive improvements in conditions from continuing rapid population declines, is what changes societys the natural trend toward conflict into a natural trend toward peace. Societies that move continuously toward peace might not have their walls knocked down.
Why is the one child per family behavior so powerful in creating peace? It creates rapidly increasing well-being universally over the whole population. Consider the following proof. A person has (or had) 2 parents, 4 grand parents, and 8 great grand parents. Each great grand parent consumed resources to feed, house, and enjoy themselves. If each generation had only one child, then that child could double consumption without increasing the familys consumptive footprint. That is, each of your 4 grand parents could consume what 8 great grand parents consumed. Your 2 parents could consume what 8 great grand parents. You could consume what 8 great grand parents consumed. Your kid could consume (given that you wife came from similar lineage) what 16 great grand parents consumed. From an individuals perspective, assuming that a generation is 30 years, you are born into a house, at 30 you have two houses, at 60 you have four and at 90 you have eight. Your expectations of ever increasing well-being should be so grand that you will not want to kill this system by fighting over material resources.
"But if the population is falling, who would support old people?" In this system people are so much better off in there later years they dont need any help. Are you planning to support your parents in their old age or are you expecting to inherit something?
Do we really need 6 billion people to make this world work? 2% of the population grows most of our food. 2% provides our utilities. For humankind to maintain the human potential and keep advancing science and arts do we really need more than a few percent of the existing population?
What is the minimum global population is another story? This story is about conflict, and behaviors that move society away rather than toward it. Few would consider paying the "one child per family" price for peace. However, after September 11th, some people see -- conditions as harsh -- expectations of future conditions as harsher, -- and past solutions, political or technological as weaker. Today some people may view having "one child per family" less expensive than enduring the costs of escalating terrorism.
We can test this theory by intervening in some tragic place on earth where conflict has been the norm for many generations (that is almost anywhere.) Consider dividing the community not along ethnic and religious lines, but along those willing to pay the price of limiting themselves to one child and those who are not.
Close the national boarder to outside immigration. Divide the nation into two states. Close the boarder between the two states so people can emigrate out of "state 1" but not emigrate into it from state 2. To be a citizen of "state 1" you have to be born to a union of two citizens from "state 1."
To be a citizen of "state 2" you have to be born to parents of either state. There are no exceptions to these rules. Visitors to either state may stay only 90 days per year.
The price for living in "state 1" is that each person parents only one child. There are no special costs for living in "state 2." After having a second child in "state 1," everyone in the family must emigrate to "state 2" which must accept them." In exchange, "state 1" will accept one couple from state 2 if they have less than two children and are willing to pay state 1s price not have any more.
State 1's" "peace price" stays in place until the population declines enough that the remaining population has adequate food, jobs, education, health care, recreation, and enough "expectations of expanding per capita well-being" to result in a no social violence condition. People would rather wait for things to get better than fight to make them better.
Implementation: After the boundary of the nation is drawn, people inside that boundary decide if they want to pay the price. Those willing to pay move to "state 1" and those not willing to pay move to "state 2." The size of "state 1 and 2" are adjusted so each person has about what they had before the move - the only difference is that the "payers" and "non-payers" do not share the same ground.
World governments, who are running this experiment as an alternative to the costs of endless conflict in that location, pay the dislocation losses and the moving expenses. It builds an impenetrable fence around the nation and between its states. The place is cleansed of any weapon bigger than a rock. It places an army outside the boarders of the new nation to protect it from any outside violent forces. And it prevents the importation of any tools for destruction.
I suggest this experiment will show us that the state with rapidly decreasing population will move toward peace. The state with constant or increasing populations will move toward conflict.