2.1 children per woman
The world birth-rate is plummeting. From 6.2 children the year 1972 to 2.4 children per woman today. No society can survive a long term decline of the birth-rate. If the birth rate is too low like it has been in Europe since 1972 and in Japan since 1975 big problems will come.
The economy will fade away. Each declining country will have large internal problems. The wealth will be spoiled. Today ´s politicians will not be able to cope with the problems because they cannot think an plan 25 years ahead. Short-sighted power men like politicians, editor-in-chiefs, powerful NGOs will block all attempts to change by using egoism. This is the typical downward spiral that we now see in Europe, Japan and some other countries.
In order to increase the birth rates it will be necessary to change the way plan their long term careers. This can be done and has been done in the US and in Australia.
One of the basic problems is that the more years of education you have the fewer children you have. Education is the overwhelming factor that influences the birth-rate. Many Empires have Disappeared due to education and egoism. Egoism usually follows intensive education.
Knowledge without knowledge is not realistic way to govern the earth.
Some examples of defunct empires are; the Etruscians, the Roman Empire and the Abbasid khalifate.
The future of the planet earth is depending if we can keep the knowledge. If education does not pay off, this world is doomed to die.
Pls read more about knowledge at the site knowledge of knowledge.