At a time of concerns about globalization and immigration, the idea of a wall along the U.S. border with Mexico appears attractive to many Americans. In addition to its direct effects, a border wall would send a political signal indicating U.S. disengagement from Mexico and opposition to further immigration from the south.

However, a border wall would be a wasteful endeavor, even if the sentiments that motivated it could be justified. Like numerous walls throughout history, the proposed border wall would probably be undermined by tunnelers. The strongest evidence for this is the fact that tunnels have already been used to bypass existing fences on the border. Some of these tunnels enable short-range clandestine movements between homes or warehouses on opposite sides of the border. Others use existing underground infrastructure, such as storm drains, to facilitate movements at greater distances.

These tunnels appear to be used exclusively for smuggling drugs, rather than people. The tunnels are expensive enough that sending migrants through them is too risky: subsequently apprehended migrants may report the tunnel's location, and the price of their passage is not enough to offset the risk of potentially losing an expensive asset. However, if the cost of a crossing went up due to overland routes becoming more challenging, tunneling would become more desirable, and there would be greater interest in cost-cutting measures.

In general, fences and walls do not prevent people from crossing boundaries; they merely slow them down. Existing fences along densely populated parts of the U.S.-Mexican border enable the Border Patrol to observe and stop people trying to get past them. Along most of the length of the border, natural obstacles including deserts, mountains and rivers already serve to slow down prospective crossers. The addition of a wall in remote country would represent a minor additional obstacle to be climbed. While networks of sensors along the wall could facilitate response by the Border Patrol, such networks could be deployed independently of a wall.

On the other hand, a 1,950-mile sensor network would be expensive, with or without a wall: It would need to be continuously maintained in remote, often harsh environments, and would be subject to vandalism. Such networks also generate false alarms due to wildlife movements and algorithmic errors. More Border Patrol agents and civil servants would be needed both to monitor the network and to respond to the alarms it would generate.

History suggests those resources would be lacking. Brent Sterling showed in his book Do Good Fences Make Good Neighbors? that defensive barriers are rarely given adequate personnel support. Maintenance and monitoring are costly, even on a small scale, such as Israel's anti-terrorist barriers with Gaza and the West Bank (two territories that are 25 and 80 miles across, respectively). Most of the 1,950 miles of the U.S. wall would probably be under-supported over time, particularly if more money went to countering terrorist threats: Not one U.S. terrorist attack has involved an illegal crossing of the Mexican border.

An alternative to a wall would be to increase the number of Border Patrol agents, a number that has already more than doubled since 2000. During the same period, the number of apprehensions by the Border Patrol has fallen by 80 percent. According to the Pew Research Center, since 2007, the number of illegal Mexican residents of the U.S. has fallen by about 20 percent, and more Mexican citizens have left the U.S. than have entered it, either legally or illegally. While the reasons for this decline in migration likely include economic and demographic shifts, increasingly effective border security is probably a factor. Of course, increasing the number of personnel along an increasingly secure border lacks the tangibility and apparent permanence of a thick layer of concrete from sea to shining sea.

The proposed wall would need to be permeated by legal entry points to allow for trade and tourism. Mexico is the U.S.' second-largest export market and third-largest trading partner, with many supply chains integrated across the frontier. Severing legitimate cross-border movements would be tremendously damaging to the U.S. economy, while also harming the Mexican economy to the point that more people would attempt to cross the border illegally.

Even if, hypothetically, a wall along the U.S.-Mexican frontier were to increase border security, it would not eliminate illegal migration. Many illegal residents of the U.S. arrived legally and then overstayed their visas. If the cost of illegal crossings increased, alternative routes to the U.S. could become economically viable, including the use of tunnels and concealment inside vehicles moving legitimate commerce.

A wall would be an expensive way of achieving a limited impact in terms of curbing illegal migration. Its main impact would be symbolic, as a harsh gesture to a neighbor on which the U.S. economy depends.

Scott Savitz is a senior engineer at the nonprofit, nonpartisan RAND Corp. Email: Scott_Savitz@rand.org