The country's immigration and naturalization laws have been subjects of episodic controversy since America's founding, but unauthorized migration only became an issue in the early 20 th century, when Congress passed the first strict restrictions on legal admissions. Unauthorized migration declined during the Great Depression and during and after World War II, when most labor migration occurred through the U.S.-Mexico Bracero program. Immigration control re-emerged as a national concern during the 1970s, when the end of the Bracero program, new restrictions on Western Hemisphere migration, and growing U.S. demand for foreign-born workers combined to cause a sharp increase in unauthorized migration flows. Congress responded in 1986 by passing the Immigration Reform and Control Act (IRCA, P.L. 99-603 ), which authorized a 50% increase in Border Patrol staffing, among other provisions. Border security has remained a persistent topic of congressional interest since then, and enforcement programs and appropriations have grown accordingly, as described in this report. Despite a growing enforcement response, however, unauthorized migration continued to increase over most of the next three decades. After 2005, unauthorized migrant apprehensions began to decline, suggesting a decrease in unauthorized migration. Apprehensions of unauthorized migrants at the U.S.-Mexico border fell from about 1.2 million in FY2005 to a 41-year low of 378,577 in 2011. Apprehensions then climbed to 479,371 in FY2014, before falling to 331,333 in FY2015. Additionally, it is estimated that after 2007 the number of unauthorized migrants living in the United States also declined. In 2014, the Obama Administration announced executive actions to "fix" the immigration system. These actions address several issues, including a security plan at the southern border. Some Members of Congress and state officials, however, disagree with the President's plan and have called on the Administration to do more to secure the border. Border security has been a recurrent theme in Congress's debate about immigration reform since 2005, and some Members of Congress have argued that Congress should not consider additional immigration reforms until the border is more secure. This report reviews efforts to combat unauthorized migration across the Southwest border in the nearly three decades since IRCA initiated the modern era in migration control. In reviewing such efforts, the report takes stock of the current state of border security and considers lessons that may be learned about enhanced enforcement at U.S. borders. The report begins by reviewing the history of border control and the development of a national border control strategy beginning in the 1990s. The following sections summarize appropriations and resources dedicated to border enforcement, indicators of enforcement outcomes, metrics for border security, and possible secondary and unintended consequences of border enforcement. The report concludes by reviewing the overall costs and benefits of the current approach to migration control and raising additional questions that may help guide the discussion of these issues in the future. Congress created the U.S. Border Patrol (USBP) within the Departments of Commerce and Labor by an appropriations act in 1924, two days after passing the first permanent numeric immigration restrictions. Numerical limits only applied to the Eastern Hemisphere, barring most Asian immigration; and the Border Patrol's initial focus was on preventing the entry of Chinese migrants, as well as combating gun trafficking and alcohol imports during prohibition. The majority of agents were stationed on the northern border. The Border Patrol became part of the new Immigration and Naturalization Service (INS) in 1933, and the INS moved from the Department of Labor to the Department of Justice in 1940. The Border Patrol's focus shifted to the Southwest border during World War II, but preventing unauthorized migration across the Southwest border remained a low priority during most of the 20 th century. Unauthorized migration from Mexico increased after 1965 as legislative changes restricted legal Mexican immigration at the same time that social and economic changes caused stronger migration "pushes" in Mexico (e.g., inadequate employment opportunities) and stronger "pulls" in the United States (e.g., employment opportunities, links to migrant communities in Mexico). Congress held hearings on unauthorized migration beginning in 1971, and after more than a decade of debate passed the Immigration Reform and Control Act of 1986 (IRCA, P.L. 99-603 ), which described border enforcement as an "essential element" of immigration control and authorized a 50% increase in funding for the Border Patrol, among other provisions. Congress passed at least 11 additional laws addressing unauthorized migration over the next two decades, 7 of which included provisions related to the border. Seventy years after it began operations, the Border Patrol developed its first formal national border control strategy in 1994, the National Strategic Plan. The plan was updated in 2004 and again in 2012. The National Strategic Plan (NSP) was developed in 1994 in response to a widespread perception that the Southwest border was being overrun by unauthorized migration and drug smuggling, and to respond to a study commissioned by the Office of National Drug Control Policy. The study recommended that the INS change its approach from arresting unauthorized migrants after they enter the United States, as had previously been the case, to focus instead on preventing their entry. Under the new approach, the INS would place personnel, surveillance technology, fencing, and other infrastructure directly on the border to discourage unauthorized flows, a strategy that became known as "prevention through deterrence." According to the 1994 INS plan, "the prediction is that with traditional entry and smuggling routes disrupted, illegal traffic will be deterred, or forced over more hostile terrain, less suited for crossing and more suited for enforcement." The strategy had four phases that began with the border patrol sectors with the highest levels of unauthorized migration activity. Phase I: San Diego, CA, and El Paso, TX, sectors; Phase II: Tucson, AZ, Del Rio, TX, Laredo, TX, and McAllen, TX, sectors; Phase III: El Centro, CA, Yuma, AZ, and Marfa, TX, sectors; and Phase IV: The northern border, gulf coast, and coastal waterways. The strategy yielded several initiatives aimed at stemming unauthorized migration and human smuggling, and interdicting drug trafficking. These initiatives included the following: Operation Gatekeeper was first initiated at the San Diego Border Patrol Sector and was later extended to the El Centro Border Patrol Sector. The initiative involved providing border patrol agents with additional resources, such as increased staffing and new technologies. Operation Safeguard was initiated at the Tucson Border Patrol Sector and was aimed at stemming unauthorized migration by pushing unauthorized migrants away from urban areas. Operation Hold the Line was initiated at the El Paso Border Patrol Sector and was aimed at the specific needs of the community. New border patrol agents were added to the area and innovative resources were deployed, including IDENT terminals. The initiative also included the installation of fences along parts of the border and other infrastructure improvements. Operation Rio Grande was initiated at the McAllen Border Patrol Sector and focused on increasing the number of border patrol agents. The implementation of Phase II and subsequent phases was to be based on the success of Phase I, with the plan describing several expected indicators of effective border enforcement, including an initial increase of border arrests and entry attempt to be followed by an eventual reduction of arrests, a change in traditional traffic patterns, and an increase in more sophisticated smuggling methods. As predicted, apprehensions within the San Diego and El Paso sectors fell sharply beginning in 1994-1995, and traffic patterns shifted, primarily to the Tucson and South Texas (Rio Grande Valley) sectors (see " Southwest Border Apprehensions by Sector "). A 1997 General Accounting Office (GAO, now called the Government Accountability Office) report was cautiously optimistic about the strategy. Congress supported the prevention through deterrence approach. In 1996, House and Senate appropriators directed the INS to hire new agents and to reallocate personnel from the interior to front line duty. And the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 ( P.L. 104-208 ) expressly authorized the construction and improvement of fencing and other barriers along the Southwest border and required the completion of a triple-layered fence along 14 miles of the border near San Diego where the INS had begun to install fencing in 1990. In the wake of the September 11, 2001, terrorist attacks, the USBP refocused its priorities on preventing terrorist penetration, while remaining committed to its traditional duties of preventing the illicit trafficking of people and contraband between official ports of entry. Shortly after the creation of DHS, the USBP was directed to formulate a new National Border Patrol Strategy (NBPS) that would better reflect the realities of the post-9/11 security landscape. In March 2004, the Border Patrol unveiled the National Border Patrol Strategy, which placed greater emphasis on interdicting terrorists and featured five main objectives: establishing the substantial probability of apprehending terrorists and their weapons as they attempt to enter illegally between the ports of entry; deterring unauthorized entries through improved enforcement; detecting, apprehending, and deterring smugglers of humans, drugs, and other contraband; leveraging "Smart Border" technology to multiply the deterrent and enforcement effect of agents; and reducing crime in border communities, thereby improving the quality of life and economic vitality of those areas. The NBPS was an attempt to lay the foundation for achieving "operational control" over the border, defined by the Border Patrol as "the ability to detect, respond, and interdict border penetrations in areas deemed as high priority for threat potential or other national security objectives." The strategy emphasized a hierarchical and vertical command structure, featuring a direct chain of command from headquarters to the field. The document emphasized the use of tactical, operational, and strategic intelligence and sophisticated surveillance systems to assess risk and target enforcement efforts; and the rapid deployment of USBP agents to respond to emerging threats. Additionally, the plan called for the Border Patrol to coordinate closely with CBP's Office of Intelligence and other federal intelligence agencies. CBP published a new Border Patrol Strategic Plan (BPSP) in May 2012 that shifted attention from resource acquisition and deployment to the strategic allocation of resources by "focusing enhanced capabilities against the highest threats and rapidly responding along the border." From an operational perspective, the 2012 plan emphasizes the collection and analysis of information about evolving border threats; integration of Border Patrol and CBP planning across different border sectors and among the full range of federal, state, local, tribal, and international organizations involved in border security operations; and rapid Border Patrol response to specific border threats. The Border Patrol's approach to border enforcement has been mirrored in broader DHS policies. In November 2005, the Department of Homeland Security announced a comprehensive multi-year plan, the Secure Border Initiative (SBI), to secure U.S. borders and reduce unauthorized migration. Under SBI, DHS announced plans to obtain operational control of the northern and southern borders within five years by focusing attention in five main areas: Increased staffing . As part of SBI, DHS announced the addition of 1,000 new Border Patrol agents, 250 new ICE investigators targeting human smuggling operations, and 500 other new ICE agents and officers. Improved detention and removal capacity . Historically, most non-Mexicans apprehended at the border were placed in formal removal proceedings. Yet backlogs in the immigration court system meant that most such migrants were released on bail or on their own recognizance prior to a removal hearing, and many failed to show up for their hearings. In October 2005, DHS announced plans to detain 100% of non-Mexicans apprehended at the border until they could be processed for removal. SBI supported this goal by adding detention capacity, initially increasing bed space by 2,000 to a total of 20,000. On August 23, 2006, DHS announced that the policy to "end catch and release" had been successfully implemented. Surveillance technology . SBI included plans to expand DHS's use of surveillance technology between ports of entry, including unmanned aerial vehicle (UAV) systems, other aerial assets, remote video surveillance (RVS) systems, and ground sensors. These tools were to be linked into a common integrated system that became known as SBI net (see " Surveillance Assets " below). Tactical infrastructure . SBI continued DHS's commitment to the expansion of border fencing, roads, and stadium-style lighting. Interior enforcement . SBI also included plans to expand enforcement within the United States at worksites and through state and local partnerships, jail screening programs, and task forces to locate fugitive migrants. Although not the subject of a formal public policy document like those discussed above, an additional component of CBP's approach to border control in recent years has been an effort to promote "high consequence" enforcement for unauthorized Mexicans apprehended at the border. Historically, immigration agents permitted most Mexicans apprehended at the border to voluntarily return to Mexico without any penalty. Since 2005, CBP has limited voluntary returns in favor of three types of "high consequence" outcomes: In general, these high consequence enforcement outcomes are intended to deter unauthorized flows by raising the costs to migrants of being apprehended and by making it more difficult for them to reconnect with smugglers following a failed entry attempt. To manage these disparate programs, CBP has designed the "Consequence Delivery System.... to uniquely evaluate each subject [who is apprehended] and identify the ideal consequences to deliver to impede and deter further illegal activity." USBP agents use laminated cards with matrices describing the range of enforcement actions available for a particular migrant as a function of the person's immigration and criminal histories, among other factors, and of the enforcement resources available in each Border Patrol sector. According to public comments by then CBP Commissioner Alan Bersin, the goal of the Consequence Delivery System, in certain sectors of the border, is to ensure that virtually everyone who is apprehended faces "some type of consequence" other than voluntary return. With the implementation of the Consequence Delivery System, the Border Patrol has initiated a system to estimate the deterrent effect of different enforcement outcomes. In particular, the Border Patrol tracks, for each of 10 different enforcement consequences, the percentage of migrants who were re-apprehended during the same fiscal year following repatriation (i.e., the recidivism rate). However, changes in recidivism rates may not be wholly attributable to differences among the consequences because the Border Patrol takes account of migrants' migration histories and other factors when assigning people to different enforcement outcomes. In a November 20, 2014, memo, Secretary Johnson "commissions" three Joint Task Forces. According to Secretary Johnson, the three Joint Task Forces will bring together personnel from CBP as well as other DHS agencies—U.S. Coast Guard (USCG), U.S. Immigration and Customs Enforcement (ICE), and U.S. Citizenship and Immigration Services (USCIS). The Joint Task Forces will also "integrate capabilities of the remaining [DHS] components as needed." According to Secretary Johnson, "Two of these task forces will be geographically based and one will be functionally focused." Joint Task Force East will cover "the Southern maritime border and approaches." Joint Task Force West will have responsibility for the southern land border and the West Coast, while an entity dubbed Joint Task Force Investigations "will focus on investigations in support of the geographic Task Forces." The memo lays out broad missions and objectives and sets a 90-day timeframe to "realign personnel and stand up headquarters capabilities within each Joint Task Force." Statutory and strategic changes since 1986 are reflected in border enforcement appropriations and in CBP's assets at the border, including personnel, infrastructure, and surveillance technology. This section reviews trends in each of these areas. Figure 1 depicts U.S. Border Patrol appropriations for FY1990-FY2015. Appropriations have grown steadily over this period, rising from $263 million in FY1990 to $1.4 billion in FY2002 (the last year before the creation of DHS), $3.0 billion in FY2010, and $3.8 billion in FY2015. The largest growth came following the formation of DHS in FY2003, reflecting Congress's focus on border security in the aftermath of 9/11. Appropriations reported in Figure 1 are only a subset of all border security funding for FY1990 to FY2015. These data do not include additional CBP sub-accounts. For example, in FY2016 funding was also provided to Headquarters Management and Administration ($1.4 billion), and Border Security Inspections and Trade Facilitation at Ports of Entry ($3.4 billion); and additional CBP accounts funding Border Security Fencing, Infrastructure, and Technology ($447 million); Automation Modernization ($829 million); Air and Marine Operations ($802 million) and Construction and Facilities Management ($340 million). A substantial portion of these accounts is dedicated to border security and immigration enforcement, as these terms are commonly used. And Figure 1 excludes border enforcement appropriations for other federal agencies—including the Departments of Justice, Defense, the Interior, and Agriculture, all of which play a role in border security—as well as funding for the U.S. federal court system. Accompanying this budget increase, Congress has passed at least four laws since 1986 authorizing increased Border Patrol personnel. USBP staffing roughly doubled in the decade after the 1986 IRCA, doubled again between 1996 and the 9/11 attacks, and doubled again in the decade after 9/11 (see Figure 2 ). In FY2015, USBP had 20,273 agents, including 17,522 posted at the Southwest border. The National Guard also is authorized to support federal, state, and local law enforcement agencies (LEAs) at the border. Basic authority for the Department of Defense (DOD, including the National Guard) to assist LEAs is contained in Chapter 18 of Title 10 of the U.S. Code, and DOD personnel are expressly authorized to maintain and operate equipment in cooperation with federal LEAs in conjunction with the enforcement of counterterrorism operations or the enforcement of counterdrug laws, immigration laws, and customs requirements. DOD may assist any federal, state, or local LEA requesting counterdrug assistance under the National Defense Authorization Act, as amended. Under Title 32 of the U.S. Code, National Guard personnel also may serve a federal purpose, such as border security, and receive federal pay while remaining under the command control of their respective state governors. National Guard troops were first deployed to the border on a pilot basis in 1988, when about 100 soldiers assisted the U.S. Customs Service at several Southwest border locations, and National Guard and active military units provided targeted support for the USBP's surveillance programs throughout the following decade. The first large-scale deployment of the National Guard to the border occurred in 2006-2008, when over 30,000 troops provided engineering, aviation, identification, technical, logistical, and administrative support to CBP as part of "Operation Jump Start." President Obama announced an additional deployment of up to 1,200 National Guard troops to the Southwest border on May 25, 2010, with the National Guard supporting the Border Patrol, by providing intelligence work and drug and human trafficking interdiction. The 2010 deployment was originally scheduled to end in June 2011, but the full deployment was extended twice (in June and September 2011). The Administration announced in December 2011 that the deployment would be reduced to fewer than 300 troops beginning in January 2012, with National Guard efforts focused on supporting DHS's aerial surveillance operations. In December 2012, DHS and the Department of Defense announced that the National Guard deployment would be extended through December 2013. In 2014 Texas Governor Greg Abbott deployed approximately 1,000 National Guard troops to the Southwest border in response to the increase in child migrants. In December 2015, Governor Abbott announced that he would extend their deployment to deal with newly increasing numbers of child migrants in the first months of FY2016. Additionally, on April 5, 2016, Alabama Governor Robert Bentley announced that he would be ordering Alabama National Guard support to the Southwest border to assist in protecting the border. He deployed one helicopter and three pilots to Marana, AZ. Border tactical infrastructure includes roads, lighting, pedestrian fencing, and vehicle barriers. Tactical infrastructure is intended to impede illicit cross-border activity, disrupt and restrict smuggling operations, and establish a substantial probability of apprehending terrorists seeking entry into the United States. The former INS installed the first fencing along the U.S.-Mexican border beginning in 1990 east of the Pacific Ocean near San Diego. Congress expressly authorized the construction and improvement of fencing and other barriers under Section 102(a) of the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 (IIRIRA), which also required the completion of a triple-layered fence along the original 14 mile border segment near San Diego. The Secure Fence Act of 2006 amended IIRIRA with a requirement for double-layered fencing along five segments of the Southwest border, totaling about 850 miles. IIRIRA was amended again by the Consolidated Appropriations Act, FY2008. Under that amendment, the law now requires the Secretary of Homeland Security to construct reinforced fencing "along not less than 700 miles of the southwest border where fencing would be most practical and effective and provide for the installation of additional physical barriers, roads, lighting, cameras, and sensors to gain operational control of the southwest border." The act further specifies, however, that the Secretary of Homeland Security is not required to install fencing "in a particular location along the international border of the United States if the Secretary determines that the use or placement of such resources is not the most appropriate means to achieve and maintain operational control over the international border at such location." As of May 2015, DHS installed 353 miles of primary pedestrian fencing, 300 miles of vehicle fencing (total of 653 miles), 36 miles of secondary fencing behind the primary fencing, and 14 miles of tertiary pedestrian fencing behind the secondary fence. The Border Patrol reportedly had identified a total of 653 miles of the border as appropriate for fencing and barriers. Figure 3 summarizes annual appropriations for tactical infrastructure (including surveillance technology) for FY1996-FY2016. Appropriations increased from $25 million in FY1996 to $298 million in FY2006, an 11-fold increase (8-fold when adjusting for inflation), and then jumped to $1.5 billion in FY2007 as DHS created a new Border Security Fencing, Infrastructure, and Technology (BSFIT) account and appropriated money to pay for the border fencing mandate in the Secure Fence Act of 2006. BSFIT appropriations decreased for seven consecutive years (from FY2007 through FY2013); however, in FY2016 BSFIT appropriations increased to $447 million (from $382 million in the previous fiscal year). The Border Patrol uses advanced technology to augment its agents' ability to patrol the border. USBP's border surveillance system has its origins in the former Immigration and Naturalization Service's (INS's) Integrated Surveillance Information System (ISIS), initiated in 1998. ISIS was folded into a broader border surveillance system named the America's Shield Initiative (ASI) in 2005, and ASI was made part of DHS's Secure Border Initiative (SBI) the following year, with the surveillance program renamed SBI net and managed under contract by the Boeing Corporation. Under all three of these names, the system consisted of a network of remote video surveillance (RVS) systems (including cameras and infrared systems), and sensors (including seismic, magnetic, and thermal detectors), linked into a computer network, known as the Integrated Computer Assisted Detection (ICAD) database. The system was intended to ensure seamless coverage of the border by combining the feeds from multiple cameras and sensors into one remote-controlled system linked to a central communications control room at a USBP station or sector headquarters. USBP personnel monitoring the control room screened the ICAD system to re-position RVS cameras toward the location where sensor alarms were tripped. Control room personnel then alerted field agents to the intrusion and coordinated the response. All three of these systems struggled to meet deployment timelines and to provide USBP with the promised level of border surveillance. DHS also faced criticism of ASI and SBI net for non-competitive contracting practices, inadequate oversight of contractors, and cost overruns. DHS Secretary Napolitano ordered a department-wide assessment of the SBI net technology project in January 2010 and suspended the SBI net contract in March 2010. The review confirmed SBI net 's history of "continued and repeated technical problems, cost overruns, and schedule delays, raising serious questions about the system's ability to meet the needs for technology along the border." DHS terminated SBI net in January 2011. Under the department's Arizona Surveillance Technology Plan, the Border Patrol deploys a mix of different surveillance technologies designed to meet the specific needs of different border regions. According to CBP officials, the department's acquisitions strategy emphasizes flexible equipment and mobile technology that permits USBP to surge surveillance capacity in a particular region, and off-the-shelf technology in order to hold down costs and get resources on the ground more quickly. In addition to these ground-based surveillance assets, CBP deploys manned and unmanned aircraft as well as marine vessels to conduct surveillance. Air and marine vessels patrol regions of the border that are inaccessible to other surveillance assets, with unmanned aerial systems (UAS) deployed in areas considered too high-risk for manned aircraft or personnel on the ground. As of October 2015, CBP's Air and Marine Operations (AMO) deployed 19 types of aircraft and three classes of marine vessels, for a total of 245 aircraft and 295 marine vessels operating from over 91 locations. For FY2014, the agency reported 90,740 flight hours (up from about 81,045 in FY2012) and 42,860 underway hours in marine vessels (down from about 47,742 in FY2012). As of March 2016, CBP operated a total of 9 UAS, up from zero in 2006. UAS accounted for 5,502 flight hours in FY2015, up from 4,611 hours in FY2014. With support from Department of Defense (DOD), CBP conducted an evaluation of two unmanned aerostat (tethered blimp) systems during the summer of 2012: the Persistent Ground Surveillance System (PGSS) and the Rapid Aerostat Initial Deployment (RAID). In addition, CBP evaluated PGSS and RAID towers, which support aerostat deployment as well as ground-based technologies. These two systems have been deployed by the military to conduct area surveillance. As a result of the evaluation, CBP concluded that these systems could provide useful support to CBP operations on the border; and CBP reportedly is working with DOD to identify opportunities to transfer ownership of aerostats returning from overseas to CBP. For 90 years, the Border Patrol has recorded the number of deportable migrants apprehended in the United States; and migrant apprehensions remain the agency's primary indicator of immigration enforcement. The agency also collects several additional measures of immigration enforcement, including unique apprehensions, migrant recidivism, and estimated turn backs and got aways. These enforcement outcomes provide insight into the state of the border, as discussed in this section, but they confront certain limitations when it comes to estimating unauthorized border inflows (see " Metrics of Border Security "). Figure 4 depicts total USBP apprehensions of deportable and removable migrants for FY1960-FY2015. Apprehensions are widely understood to be correlated with unauthorized flows, and the data in Figure 4 reflect historical trends in unauthorized migration (see " Border Patrol History and Strategy "). Apprehensions were very low in the 1960s, but climbed sharply in the two decades after 1965. Apprehensions reached an all-time high of 1.7 million in 1986 and again in 2000, and an average of more than 1.2 million apprehensions per year were recorded 1983-2006, reflecting high levels of unauthorized migration throughout this period. As Figure 4 also illustrates, apprehensions have fallen sharply since 2000, and particularly since 2006. The 340,252 apprehensions observed in 2011 were the lowest level since 1971, and the 364,768 apprehensions in 2012 were the second-lowest level since that time. Falling apprehensions may reflect fewer unauthorized inflows between 2006 and 2012, though the degree to which reduced inflows were a result of effective enforcement versus other factors like the recent U.S. economic downturn remains subject to debate (see " How Secure is the U.S. Border? "). However, apprehensions increased from FY2012 to FY2014, rising to 486,651 in FY2014, before falling to 337,117 in FY2015. The overwhelming majority of apprehensions take place along the Southwest border. In FY2015, the Border Patrol apprehended 331,333 migrants along this border, a decrease of 148,038 from the previous year. Forty-four percent of these apprehensions occurred in the Rio Grande Valley Sector (see discussion below). Figure 5 depicts apprehensions along the Southwest border for FY1990-FY2015, broken down by certain Border Patrol sectors. The sector-specific apprehension pattern generally adheres to the predictions of the 1994 National Strategic Plan. Increased enforcement in the El Paso and San Diego sectors was associated with high apprehensions in those sectors during the early 1990s, and then with falling apprehensions by the middle of the decade. Apprehensions in the San Diego and El Paso sectors remained well below their early-1990s levels throughout the following decade—findings that suggest border enforcement in those sectors has been broadly effective. Falling apprehensions in San Diego and El Paso during the late 1990s initially were more than offset by rising apprehensions in the Tucson, AZ, sector and other border locations, including the Laredo and Del Rio, TX, sectors. In FY2014, apprehensions in Tucson fell to their lowest level since 1993. Apprehensions in the Rio Grande Valley increased from FY2011 to FY2014, before falling in FY2015. Nonetheless, apprehensions in the Rio Grande Valley now account for more than a third of Southwest border apprehensions. Thus, since the initiation of the prevention through deterrence approach in the mid-1990s, it appears that success in San Diego and El Paso may have come at the expense of Tucson and other sectors. Overall apprehensions data record apprehension events , and therefore count certain individuals more than once if they enter and are apprehended multiple times. Since 2000, the Border Patrol also has tracked the number of unique subjects the agency apprehends per year by analyzing biometric data (i.e., fingerprints and digital photographs) of persons apprehended. Figure 6 depicts annual Southwest border recidivism rates, which USBP has tracked since 2005. The Border Patrol defines the annual recidivism rate as the percentage of unique subjects apprehended more than once in a given fiscal year. A goal of the Consequence Delivery System has been to deter migrants from re-entering—that is, to reduce recidivism. As Figure 6 illustrates, recidivism rates increased slightly between FY2005 (25%) and FY2007 (29%), but have fallen since that time, reaching 14% in FY2015. Border Patrol stations and sectors estimate the number of unauthorized entrants who successfully travel to the U.S. interior and who USBP ceased pursuing, or "got aways." Stations and sectors also estimate "turn backs," the number of people who cross the border unauthorized but then cross back to Mexico. USBP uses the sum of got aways, turn backs, and apprehensions to estimate the total number of known unauthorized entries . The agency has used these data since 2006 to inform tactical decision making and to allocate resources across Southwest border sectors, but the Border Patrol has not published them or viewed them as reliable metrics of border security because of challenges associated with measuring got aways and turn backs across different border sectors. While the Border Patrol has data on various enforcement outcomes , these enforcement data were not designed to measure overall border security or unauthorized inflows. Furthermore, enforcement data depends on enforcement resources. In general, USBP enforcement outcomes (e.g., apprehensions, estimated got aways) are a function of (1) the underlying unauthorized flows and (2) the agency's ability to detect such flows. Enforcement data alone cannot disentangle these two factors. As a result, enforcement data may tend to overestimate unauthorized flows where resources are robust, and to under-estimate such flows where resources are scarce. Thus, DHS officials have testified that current enforcement data do not offer a suitable metric to describe border security. Given the limits of existing border enforcement data, over time, DHS and USBP have developed different metrics for estimating unauthorized border flows and describing border security. Border security metrics are used at both the strategic and operational levels. At the strategic level, DHS uses metrics to understand its ability to meet border security objectives. The department is currently working to develop a new metric of border security, the unauthorized flow of migrants into the United States . The estimate will be generated through the use of the repeated trials method, also known as the capture-recapture method. Additionally, DHS also reports two other metrics of border security on its annual performance reports. First, the percentage of people apprehended multiple times along the Southwest border, or the recidivism rate, is used to capture USBP's ability to deter migrants from re-entering the United States. Second, the rate of interdiction effectiveness along the Southwest border between ports of entry , or the effectiveness rate, measures USBP's ability to apprehend unauthorized migrants. At the operational level, USBP conducts risk assessments that use a range of metrics. Its methodology, "State of the Border Risk Methodology," estimates the magnitude of risk through the use of intelligence information, a detailed awareness of threats at the border, and a standardized measure of risk. Though these assessments are not used as metrics themselves, USBP's methodology monitors certain metrics at the sector level that may be able to speak to USBP's performance (such as the recidivism rate and effectiveness rate). USBP conducts these risk assessments at the sector level and employs them to make day-to-day decisions with regard to how it uses its resources. While no single metric accurately and reliably describes border security, most analysts agree, based on available data, that the number of unauthorized border crossers fell sharply between about 2005 and 2011, with some rise in unauthorized flows from 2012 to 2014, and a decrease in 2015. This conclusion is supported by key Border Patrol enforcement data described above, including the drop in total apprehensions and the drop in estimated got aways and total estimated known entries across eight out of nine Border Patrol sectors. Survey data confirm an apparent drop in unauthorized inflows, and measure such effects away from the border. For example, according to data collected by the Princeton Mexican Migration Project, an average of about 2% of all Mexican men initiated a first unauthorized trip to the United States each year between 1973 and 2002; but that percentage has fallen sharply since 2002, to below 0.4% in 2008-2011. Another study found that the annual probability of a Mexican individual taking an initial unauthorized trip to the United States averaged .009 (ranging from .003 to .014) from 1970 to 2007. Estimates of the unauthorized population in the United States report drops of about 1 million unauthorized migrants, from approximately 12.2 million in 2007 to 11.3 million in 2014. Furthermore, the Pew Hispanic Center estimates that net (i.e., northbound minus southbound) migration between Mexico and the United States has resulted in a net loss of 140,000 from 2009 to 2014, meaning more Mexicans have returned to Mexico than have migrated to the United States. According to GAO's analysis of Border Patrol metrics, eight out of nine Border Patrol sectors (all except the Big Bend sector) showed improved effectiveness rates between FY2006 and FY2011. In the Tucson sector, the main focus of GAO's analysis, the effectiveness rate improved from 67% to 87% during this period. The San Diego, El Centro, Yuma, and El Paso sectors all had effectiveness rates in FY2011 of about 90%; the Del Rio and Laredo sectors (along with Tucson) had rates above 80%; and the Big Bend and Rio Grande Valley sectors had rates between 60% and 70%. Border enforcement is only one of several factors that affect unauthorized migration. Thus, if unauthorized entries indeed fell after 2006, to what degree is this change attributable to enforcement versus other developments, such as the U.S. economic downturn since 2007, and/or economic and demographic changes in Mexico and other countries of origin? Disentangling the effects of enforcement from other factors influencing migration flows is particularly difficult in the current case because many of the most significant new enforcement efforts—including a sizeable share of new border enforcement personnel, most border fencing, new enforcement practices at the border, and many of the new migration enforcement measures within the United States—all have occurred at the same time as the most severe recession since the 1930s. Nonetheless, the drop in recidivism rates suggests that an increasing proportion of migrants are being deterred by CBP's enforcement efforts. For example, one study found that border enforcement did not affect the likelihood of Mexican migrants making their first unauthorized trip to the United States but that it does have a pronounced effect on their likelihood of making additional trips. Surveys of unauthorized migrants repatriated to Mexico in 2005 and 2010 also suggest that enforcement is increasingly likely to deter future immigration. Furthermore, academic research from 2012 provides evidence that border enforcement has contributed to a reduction in unauthorized flows. These findings are noteworthy, in part, because they contradict earlier academic research, much of which found that border enforcement had a limited impact or even was counter-productive when it came to migration control efforts (also see " Migration Flows: "Caging" Effects and Alternative Modes of Entry "). This research suggests that the recent build-up in immigration enforcement at the border and within the United States may have had a greater deterrent effect on unauthorized migration than earlier efforts. Nonetheless, some uncertainty will remain about the true level of border security as long as U.S. employment demand remains below historic levels. The preceding discussion includes estimates of what may be described as the primary costs and benefits of border enforcement, defined in terms of congressional appropriations and deployment of enforcement resources on one hand, and migrant apprehensions and other indicators of successful enforcement on the other. A comprehensive analysis of the costs and benefits of border enforcement policies may also consider possible unintended and secondary consequences. Such consequences may produce both costs and benefits—many of which are difficult to measure—in at least five areas: border-area crime and migrant deaths, migrant flows, environmental impacts, effects on border communities, and U.S. foreign relations. Unauthorized border crossing is associated with a certain level of border crime and violence and, in the most unfortunate cases, with deaths of unauthorized border crossers and border-area law enforcement officers. Unauthorized migration may be associated with crime and mortality in several distinct ways. First, unauthorized migration is associated with crime—apart from the crime of unauthorized entry—because some unauthorized migrants contract with immigrant smugglers and because unauthorized migrants may engage in related illegal activity, such as document fraud. Yet fear of the police may make unauthorized migrants less likely to engage in other types of criminal activity, and research on the subject finds low immigrant criminality rates, especially when accounting for education levels and other demographic characteristics. Second, unauthorized border crossers face risks associated with crossing the border at dangerous locations, where they may die from exposure or from drowning. Border enforcement therefore may affect crime and migrant mortality in complex ways. On one hand, the concentration of enforcement resources around the border may exacerbate adverse outcomes by making migrants more likely to rely on smugglers. The 1994 National Strategic Plan predicted a short-term rise in border violence for these reasons. On the other hand, to the extent that enforcement successfully deters unauthorized crossers, such prevention should reduce crime and mortality. The concentration of law enforcement personnel near the border may further enhance public safety and migrant protection, especially where CBP has made a priority of protecting vulnerable populations. The empirical record suggests that there is no significant difference in the average violent crime rate in border and non-border metropolitan areas. Indeed, the border cities El Paso, TX, and San Diego, CA, are regularly listed among the safest large cities in the country based on their rankings among similarly sized cities in the Federal Bureau of Investigation's Uniform Crime Report. The specific impact of border enforcement on border-area crime is unknown, however, because available data cannot separate the influence of border enforcement from other factors. With respect to mortality, available data about migrant deaths along the Southwest border are presented in Figure 7 . The figures come from academic research based on local medical investigators' and examiners' offices in California, Arizona, New Mexico, and Texas between 1985 and 1998 (the University of Houston's Center for Immigration Research, CIR); Mexican foreign ministry and Mexican media counts compiled by the American Civil Liberties Union of San Diego; and data compiled by DHS based on bodies recovered on the U.S. side of the border. All three data sources reflect known migrant deaths, and therefore likely undercount actual migrant deaths since some bodies may not be discovered. Additionally, U.S. data sources generally do not include information from the Mexican side of the border and therefore further undercount migration-related fatalities. As Figure 7 illustrates, data from the CIR indicate that known migrant deaths fell from a high of 344 in 1988 to a low of 171 in 1994 before climbing back to 286 in 1998. According to DHS data, known migrant deaths climbed from 249 in 1999 to 492 in 2005, and averaged 431 deaths per year in 2005-2009. DHS's count fell to an average of 369 per year in 2010-2011, but increased to 471 in FY2012 before falling again to 445 in FY2013 and to a low of 240 in FY2015. From FY1998 to FY2015, 6,571 migrants have died, averaging 365 deaths a year. The ACLU found that known migrant deaths increased from just 80 per year in 1994-1997 to 481 per year in 1998-2008. The apparent increase in migrant deaths in the past decades is noteworthy in light of the apparent decline in unauthorized entries during the same period. These data offer evidence that border crossings have become more hazardous since the "prevention through deterrence" policy went into effect in the 1990s, though (as with crime) the precise impact of enforcement on migrant deaths is unknown. With unauthorized border crossing becoming more dangerous and more expensive, some unauthorized migrants appear to have adapted their behavior to avoid crossing the border via traditional pathways. Most notably, social science research suggests that border enforcement has had the unintended consequence of encouraging unauthorized migrants to settle permanently in the United States rather than working temporarily and then returning home, as was more common prior to the mid-1980s. The primary evidence for this so-called "caging" effect is that unauthorized migrants appear to be staying longer in the United States and raising families here more often rather than making regular trips to visit families that remain in countries of origin. One study found that, prior to 1986, the average probability of return to Mexico for authorized migrants was .52 and that for unauthorized migrants it was .55. However, in 2006 the probability of return for authorized migrants increased to 1.0, while unauthorized migrants' probability of return dropped to .21. Furthermore, the study found that "each million dollar increase in the real value of the [Border Patrol's] budget reduces the odds of yearly return migration by 44%." Although other factors also likely contribute to these changes, survey results appear to confirm that border enforcement has been a factor behind these longer stays. A second unintended consequence of enhanced border enforcement between ports of entry may have been an increase in unauthorized entries through ports of entry and other means. According to UCSD Mexico Migration Field Research Program research, unauthorized Mexican migrants from one community in Mexico interviewed in 2009 used six different methods to enter the United States, with one in four such unauthorized migrants passing through a port of entry by using borrowed or fraudulent documents or by hiding in a vehicle. Based on three different surveys conducted between 2008 and 2010, UCSD researchers found that the probability of being apprehended while passing through a port of entry without authorization was about half as high as the probability of being apprehended while crossing between the ports. CBP's Passenger Compliance Examination (COMPEX) System reportedly detects very little unauthorized migration through ports of entry, however. There is also anecdotal evidence that unauthorized migrants have recently turned to maritime routes as alternative strategies to cross the U.S.-Mexican border. A third set of potential unintended consequences concern the effect of border enforcement on the environment. As with the effects of enforcement on border crime and violence, the effects of enforcement on the environment are complex because they reflect changes in migrant behavior and the secondary effects of enforcement per se. On one hand, many unauthorized border crossers transit through sensitive environmental areas, cutting vegetation for shelter and fire, potentially causing wildfires, increasing erosion through repeated use of trails, and discarding trash. Thus, to the extent that border enforcement successfully deters unauthorized flows, enforcement benefits the environment by reducing these undesirable outcomes. On the other hand, the construction of fencing, roads, and other tactical infrastructure may damage border-area ecosystems. These environmental considerations may be especially important because much of the border runs through remote and environmentally sensitive areas. For this reason, even when accounting for the possible environmental benefits of reduced unauthorized border flows, some environmental groups have opposed border infrastructure projects because they threaten rare and endangered species as well as other wildlife by damaging ecosystems and restricting the movement of animals, and because surveillance towers and artificial night lighting have detrimental effects on migrant birds. Although economists disagree about the overall economic impact of unauthorized migration, unauthorized migrants may impose a number of costs at the local level, including through their use of schools and other public programs. Some are also concerned that unauthorized migration undermines the rule of law. For these reasons, successful border enforcement may benefit border communities by reducing unauthorized inflows. Yet some business owners on the Southwest and Northern borders have complained that certain border enforcement efforts threaten their economic activities, including farming and ranching activities that are disrupted by the deployment of USBP resources to the border and commercial activities that suffer from reduced regional economic activity. More generally, some people have complained that the construction of barriers divides communities that have straddled international land borders for generations. Some people have raised additional concerns about the effects of border enforcement on civil rights. Some residents of Southwest and Northern border communities see enhanced border enforcement as leading to racial profiling and wrongful detentions. On top of this general concern, some people argue that Operation Streamline raises additional questions about whether migrants receive adequate legal protections during fast-tracked criminal procedures. And some have argued that mistreatment and abuse are widespread in CBP detention facilities. An additional concern that some have raised about CBP's focus on high consequence enforcement is the possibility that focusing scarce judicial and prosecutorial resources on immigration enforcement diverts attention from more serious crimes. A 2013 Justice Department study found that the number of immigration defendants in federal courts increased 664% between 1995 and 2010 (from 5,103 to 39,001); and that immigration cases accounted for 60% of the overall increase in pretrial detentions during that period. More generally, immigration offenders accounted for 46% of federal arrests in 2010, outnumbering all other crimes and up from 22% a decade earlier. Additionally, a 2008 study by the Administrative Office of the U.S. Courts found that while Congress had provided short-term funding to allow the courts to respond to increased prosecutions, the courts faced a shortage of suitable courthouse and detention facilities in some border locations. What are the effects of U.S. border enforcement policies on U.S. relations with its continental neighbors, Mexico and Canada? The United States and Canada have a strong record of collaborative border enforcement, including through 15 binational, multi-agency Integrated Border Enforcement Teams (IBETs) operating at 24 locations at and between U.S.-Canadian ports of entry. In February 2011, President Obama and Prime Minister Harper signed the Beyond the Border declaration, which described their shared visions for a common approach to perimeter security and economic competitiveness; and the countries released an Action Plan on December 7, 2011, to implement the agreement. While some Canadians have raised objections to some of the information sharing and joint law enforcement provisions of the agreement, border enforcement between the ports has not been identified as a significant source of bilateral tension. The United States and Mexico also cooperate extensively on border enforcement operations at the Southwest border. Yet immigration enforcement occasionally has been a source of bilateral tension. And with Mexicans being the most frequent target of U.S. immigration enforcement efforts, some Mexicans have expressed concerns about the construction of border fencing, the effects of border enforcement on migrant deaths, and the protection of unaccompanied minors and other vulnerable groups, among other issues related to immigration enforcement. The United States has focused substantial resources along its land borders to prevent and control unauthorized migration since the 1980s, with investments in personnel, fencing, and surveillance assets all up significantly in the post 9/11 period, in particular. Since 2005, CBP also has transformed its approach to managing enforcement outcomes, through its Consequence Delivery System. Measuring the effects of border enforcement is difficult. On one hand, after reaching a high point in 2000, Border Patrol apprehensions fell sharply in 2007-2011, reaching a 42-year low in FY2011. Apprehensions then increased from 2012 to 2014, before falling in 2015. The Border Patrol's IDENT database also indicates a declining proportion of migrants are apprehended more than once (i.e., recidivism is down). Estimates based on enforcement and survey data and accounting for estimated apprehension and deterrence rates suggest that total unauthorized inflows in 2009-2011 were well below levels observed in the two decades after IRCA's passage, but that unauthorized inflows increased somewhat in 2012. On the other hand, there is also some evidence that migrants have adapted to more difficult conditions at the border by using other means to enter the United States and by remaining longer. A comprehensive accounting also may consider various potential unintended consequences of border enforcement on the civil rights of legal residents and U.S. citizens in the border region, on migrants' human rights, on the quality of life in border communities, on the environment and wildlife, and on U.S.-regional relations. What do these findings mean for Members of Congress who oversee border security and immigration policy? Especially in light of current fiscal constraints, some Members of Congress may evaluate future border enforcement in terms of expected returns on America's investments, and they may consider the possibility that certain additional investments at the border may be met with diminishing returns. Border infrastructure may offer an example: with 651 miles of fencing and barriers already in place along the Southwest border, each additional mile would be in ever more remote locations, and therefore more expensive to install and maintain and likely to deter fewer unauthorized migrants. Similarly, some Members of Congress may question the concrete benefits of deploying more sophisticated surveillance systems across the entire northern and southern borders, including vast regions in which too few personnel are deployed to respond to the occasional unauthorized entry that may be detected. Deciding how to allocate border resources therefore requires a clear definition of the goals of border security. Zero admissions of unauthorized migrants may not be a realistic goal when it comes to migration control, as noted above, and is a higher standard than is expected of most law enforcement agencies. While this report focuses on migration control at U.S. borders, border security also encompasses the detection and interdiction of weapons of mass destruction (WMD), narcotics, and other illicit goods; policies to combat human trafficking; and other security goals. These diverse goals are often conflated in an undifferentiated debate about "border security"; but each of these goals may suggest a different mix of border investments, as well as different metrics and different standards for successful enforcement outcomes. Should policies to prevent unauthorized migration be held to the same standards as policies to prevent the entry of WMDs, for example? Regardless of how these questions are answered in principle, debates about immigration control and border security may benefit from better metrics of border security and unauthorized migration, and from a more analytical approach to program design. The Border Patrol has taken a step in this direction by analyzing recidivism data as a function of different enforcement outcomes through its Consequence Delivery System. This report also identifies several metrics for measuring border security, all of which have advantages and disadvantages. In the context of immigration policy and a possible immigration reform bill, Members of Congress may choose to focus on the total number of unauthorized migrants in the United States, in addition to border flows, since border enforcement is just one of many factors (along with interior enforcement, visa policies, etc.) influencing the size of the unauthorized population, and because more is known about the population number than about border flows.

Summary: Border enforcement is a core element of the Department of Homeland Security's effort to control unauthorized migration, with the U.S. Border Patrol (USBP) within the U.S. Customs and Border Protection (CBP) as the lead agency along most of the border. Border enforcement has been an ongoing subject of congressional interest since the 1970s, when unauthorized immigration to the United States first registered as a serious national problem; and border security has received additional attention in the years since the terrorist attacks of 2001. Since the 1990s, migration control at the border has been guided by a strategy of "prevention through deterrence"—the idea that the concentration of personnel, infrastructure, and surveillance technology along heavily trafficked regions of the border will discourage unauthorized migrants from attempting to enter the United States. Since 2005, CBP has attempted to discourage repeat unauthorized migrant entries and disrupt migrant smuggling networks by imposing tougher penalties against certain unauthorized migrants, a set of policies eventually described as "enforcement with consequences." Most people apprehended at the Southwest border are now subject to "high consequence" enforcement outcomes. Across a variety of indicators, the United States has substantially expanded border enforcement resources over the last three decades. Particularly since 2001, such increases include border security appropriations, personnel, fencing and infrastructure, and surveillance technology. In addition to increased resources, the USBP has implemented several strategies over the past several decades in an attempt to thwart unauthorized migration. In 2014, the Obama Administration announced executive actions to "fix" the immigration system. These actions address several issues, including a revised security plan at the southern border. The Border Patrol collects data on several different border enforcement outcomes; this report describes trends in border apprehensions, recidivism, and estimated "got aways" and "turn backs." Yet none of these existing data are designed to measure unauthorized border flows or the degree to which the border is secured. Thus, the report also describes methods for estimating border security at the strategic and operational levels. Drawing on multiple data sources, the report reviews the state of border security. Robust investments at the border were not associated with reduced unauthorized inflows during the 1980s and 1990s, but a range of evidence suggests a substantial drop in unauthorized inflows from 2007 to 2011, followed by a rise from 2012 to 2014 and a decrease in 2015. Enforcement, along with the 2007 economic downturn in the United States, likely contributed to the drop in unauthorized migration, though the precise share of the decline attributable to enforcement is unknown. Enhanced border enforcement also may have contributed to a number of secondary costs and benefits. To the extent that border enforcement successfully deters unauthorized entries, such enforcement may reduce border-area violence and migrant deaths, protect fragile border ecosystems, and improve the quality of life in border communities. But to the extent that migrants are not deterred, the concentration of enforcement resources on the border may increase border area violence and migrant deaths, encourage unauthorized migrants to find new ways to enter and to remain in the United States for longer periods of time, damage border ecosystems, harm border-area businesses and the quality of life in border communities, and strain U.S. relations with Mexico and Canada.