Most people say each couple should have two children to replace themselves to obtain a Zero Population Growth. However, I don't think that is accurate.
If I was Adam and my wife was Eve and we started off on the world today, the earth's population would be 2.
If my wife and I had two children at age 20; my children would have two children by the time I am forty; therefore, the world's population would double to 4.
Their children would have two more by the time I am 60 so the population would be six.
Then they would have two more by the time I died at 80 at the average age span of 80 so they would finally replace me and my wife, but the population would be six or triple with each couple only having two children.
If our world's population was at equilibrium, then it could be sustained at 2 children per couple; but as our population continues to grow, wouldn't we need to be under 2 children per couple to initially slow the growth or die quicker or have children later in life?
Scientists have looked at replacement fertility rates, mortality rates, etc., but my math is much easier. What do you think?