Border Walls, Refugees and Terrorism

Are physical walls between countries effective at reducing terrorist attacks? Despite their prohibitive cost and political divisiveness, walls between contiguous countries have recently proliferated as extreme forms of border control.  Their effectiveness at mitigating  the alleged refugee-terrorism nexus, however, is largely unknown. We use the most updated dataset on walls between adjacent countries worldwide (1975-2016), and rely on the staggered construction of walls across some country- dyads over time as an identification strategy. Our findings reveal strong backfiring effects over time, whereby attacks substantially increase after the erection of a wall. Our difference-in-differences  analyses and case studies of the walls between Saudi Arabia-Iraq (2014) and Israel-Egypt (2010) reveal two  main mechanisms.  First, out-group attitudes among both separated communities significantly deteriorate after the construction of the wall. Second, the opportunity to recruit and mobilize the aggrieved community increases  as the refugee stock from contiguous countries hosting transnational terrorist organizations is larger.