Illegal immigration outcomes on the U.S. southern border.

• Author: Roberts, Bryan W.

The state of U.S. border security has been a contentious public issue for many decades. It has not been possible to establish a coherent definition of border security, let alone measure and evaluate it. In recent years, my research colleagues and I have led an effort to define what core border security measures are and to estimate their actual values. This article reviews the key results of that effort with respect to illegal immigration across the U.S. land border with Mexico, which has been perhaps the single most controversial border security issue. Although we now have a set of credible estimates that can inform policy making and public debate, the U.S. government continues to face major challenges in reporting these measures and establishing credibility with the American public. This article concludes by offering some constructive recommendations to the government on how it could do better.

What Do Americans Believe about Illegal Immigration and Border Security?

Although many experts and analysts believe that illegal immigration has fallen significantly in recent years, a majority of the American public does not. Table 1 summarizes the results of a 2015 poll that asked if illegal immigration has risen, fallen, or stayed the same over the past few years. Most people believed that it has increased, and this is true regardless of party or political affiliations as well as socio-demographic characteristics. Polls conducted over the last 15 years show that large majorities of Americans believe that U.S. borders are not "secure" and that the government could be doing more to increase border security with respect to illegal immigration. (1)

Given these widespread perceptions, it is not at all surprising that border security has remained an intense focal point of concern and controversy. This has been driven at least in part by a complete failure of the U.S. government to publish credible border security measures that the public trusts and tell them what they really want to know about the state of border security.

Border Security Measures

American taxpayers pay for border security in order to enforce U.S. laws at the border. With respect to illegal immigration, those laws require preventing entry of those without lawful permission to enter. Therefore, the core strategic measure of border enforcement success is the number of unauthorized migrants who escape detection and successfully enter the United States illegally. (2) I will refer to this measure as the number of successful illegal entries.

Measures also should be reported on law enforcement outcomes that play important roles in determining how many migrants attempt illegal entry, and the degree to which they are deterred. One obvious measure is the probability of apprehension--the chance (on average) that someone attempting illegal entry is caught. Another measure is the rate of at-the-border deterrence--the chance that someone who has been caught and returned to their home country gives up and goes home rather than tries again to enter illegally. These variables affect a migrant's initial decision to come to the border in the first place in order to attempt illegal entry. People who know there is a very high chance of being caught, and a very high chance of giving up after being caught, will be less likely to make an initial illegal entry attempt.

There is a strong tendency in the border security debate to describe the border as either "secure" or "insecure." This is misguided, because the state of border security is never a black-or-white situation. If a border were perfectly sealed, then the number of successful illegal entries would be zero, and the probability of apprehension would equal 100 percent. If the border was perfectly open, then all entries would by definition be legal, and the probability of apprehension would be irrelevant, because there would be no border enforcement. Because these two states never actually apply in the real world, it makes more sense to estimate the values of the three core measures highlighted above and try to understand what they imply about the state of border security. These estimates are critically important for both informing public debate and helping government agencies understand how their policies and programs have affected outcomes and how they might need to be changed.

Estimates of Border Security Measures for the U.S. Southern Border

There are three basic approaches that have been used to try to estimate the above border security measures (see Whitley 2012):

* Known Floiv-Based Estimates. This approach measures successful illegal entries by observations of law enforcement agents. It relies on evidence obtained through cameras and other sensors as well as tracking detections by agents in the field ("sign-cutting"). The Border Patrol refers to tins estimate as "gotaways." It also estimates "turnbacks"--those who enter illegally but then decide to return to Mexico before being apprehended. "Known flow" data comprises gotaways, turn-backs, and apprehensions. Using this data, the Border Patrol calculates the interdiction effectiveness rate (IER), which is the ratio of apprehensions plus turnbacks to the sum of apprehensions, turnbacks, and gotaways. The IER is intended to function as a proxy for the probability of apprehension.

* Survey Based Estimates. This approach is based on collecting data from migrants about their past illegal trips across the border. The probability of apprehension can be calculated from the number of reported apprehensions. The probability of at-border deterrence can be calculated from the number reporting they were ultimately unsuccessful in entering. The number of successful illegal entries can then be estimated by combining the probability of apprehension estimate with the number of apprehensions in a simple mathematical formula. Data from the Mexican Migration Project survey has been used to make these estimates since the early 1990s. (3)

* Analytically Based Estimates. This approach is based upon the repeat trials model (RTM) of the illegal entry process. Under the RTM, a migrant attempts illegal entry, is either caught or not caught, is returned to Ills or her home country after being subjected to any consequence if caught, and decides after his or her return to either continue attempting illegal entry or give up. If the migrant decides to continue, another round of this process takes place. It can be shown mathematically that under this process, the probability of apprehension is a simple function of the probability of at-border deterrence and the recidivist ratio, which is the ratio of apprehensions after the first one to all apprehensions. The first application of the RTM to estimate the probability of apprehension and number of successful illegal entries was published in 1990, and the approach was applied again using better data in a 2006 study. (4)

Until recently, the U.S. government did not publicly report estimates for any key border security measure. In 2012, the U.S. Department of Homeland Security (DHS) permitted publication of known-flow data, and it currently publishes values for the IER in its annual performance report. Values for gotaways have never been officially published by DHS, although values for the period 2006-2013 have been released through other channels. (5) Publishing measures that are based on known-flow data is an important first step. However, known-flow data are fundamentally flawed for estimating border security measures.

In January 2015, my colleague John Whitley of the Institute for Defense Analyses (IDA) and I were asked by DHS Secretary...