Another thing I've learned is that far too many parents take the statement "I don't want kids" as a personal challenge to moralize and condescend to people without children.
Na, I think it has more to do with either projecting ones own wishes onto somebody else or maybe even talk somebody down so that you can feel better if you don't are completely happy with the current situation.
I find it much more interesting to ask why somebody wants no kids. That gives much more insight into the person's mind than when you try to persuade them with your own experiences and produces much better followup talk opportunities.
Anyway, I'm curious what you think it might be evolutionary advantageous. On the face of it, if others have less kids, that would seem advantageous to your own kids.