As a kid, I was constantly subjected to fear-mongering on population growth, which was not only out of control, but certain to lead to widespread conflict, political repression, and freakish efforts at human survival. ("Soylent Green," anyone?) Now, in my middle years, I find myself increasingly assaulted with the opposite "dangers": too few babies, and a rapidly and unevenly aging world. Somehow the dire predictions of what the consequences will be have remained the same.
Funny how that works!
Some things we know intuitively from our history, both personal and global. An easy example: In the modern world, single men facing lowered prospects for marriage inside their clan -- however defined -- will get desperate enough to marry outside of it.