I am sometimes asked, "How do you know there won't be a war tomorrow (or a genocide, or an act of terrorism) that will refute your whole thesis?" The question misses the point of this book. The point is not that we have entered an Age of Aquarius in which every last earthling has been pacified forever. It is that substantial reductions in violence taken place, and it is important to understand them. Declines in violence are caused by political, economic, and ideological conditions that take hold in particular cultures at particular times. If the conditions reverse, violence could go right back up.