Borders constitute the international system of states. Accordingly, states will, from time to time, take assertive measures to secure the border, with among the most aggressive strategies being the construction of physical barriers, which we refer to as "border walls." Using original data on man-made border wall construction from 1800 to 2014, we theorize and find that in many cases, wall construction is about economic security. Significant economic disparities between the states will create incentives to illegally transport people or move goods readily available in the poorer country but highly regulated in the richer country. We find that economic disparities have a substantial and significant impact on the presence of a physical wall that is independent of formal border disputes and concerns over instability from civil wars in neighbors. In other words, "prominent examples such as the Maginot Line", constructed largely out of fear of attack, is an exception, not the exemplar, of the reasons states construct border walls.