Why Do Countries Build Walls?

Why do countries build fences and walls at their border and under what conditions are those walls and fences likely to work as intended? These questions are obviously topical right now, with the US-Mexico border a hot button issue in the US presidential election; and the Syrian refugee crisis dominating discussion the Europe  

But fences and their effectiveness have largely remained off the radar of any rigorous academic study. Until now. In the most recent edition of the journal International Security, political scientists Ron Hassner and Jason Wittenberg of UC Berkeley compiled what is the first-ever dataset of what they called "fortified boundaries" constructed between countries since 1945. 

 
Ron Hassner is on the line with me to discusses their study and the implications of some of their key findings, including the fact that we are in the midst of a fortified boundary building boom and why the religion of a country seems to make a difference in whether or not the country decides to build a border fence.