Bridger Prince Joined Nov 10, 2005 Messages 318 Aug 23, 2011 #1 It doesn't seem to be population or age, perhaps a combination of both? I can't figure it out :\
K KrikkitTwo Immortal Joined Apr 3, 2004 Messages 12,418 Aug 23, 2011 #2 AFAIK Population factor+ Tech factor+ Buildings
vexing knows Joined Dec 24, 2010 Messages 2,668 Aug 23, 2011 #3 http://forums.civfanatics.com/showpost.php?p=10505723&postcount=3