I have no data to back this up, but I recall there being a study that showed that the warmer the climate, the more hostile the civilizations. I'll go digging for it -- Not sure about population density though. Animals in captivity tend to get more hostile as their enclosure spaces are smaller.
"I think I've said this before on hubski... I believe in god. And I think he believes in us and wants us to push limits when it comes to understanding science and the universe." This is encouraging for me to read, I hope that more people adopt this type of religiosity. Why would evolution and religion need to be at odds?
The point seems to be that people find grievances, whether it's over "holy lands" or Hatfields and McCoys, there's always a squabble. Get 6.9 billion people on top of one another and those squabbles will turn in to battles pretty quickly.