The number of foreign-born people residing in the United States (an estimated 40 million) is at the highest level in our history and, as a portion of the U.S. population, has reached a percentage (12.9%) not seen since the early 20 th century. The actual number of unauthorized aliens in the United States is unknown. The three main components of the unauthorized resident alien population are (1) aliens who overstay their nonimmigrant visas, (2) aliens who enter the country surreptitiously without inspection, and (3) aliens who are admitted on the basis of fraudulent documents. In all three instances, the aliens are in violation of the Immigration and Nationality Act (INA) and subject to removal. The last major law that allowed unauthorized aliens living in the United States to legalize their status was the Immigration Reform and Control Act (IRCA) of 1986 ( P.L. 99-603 ). Generally, legislation such as IRCA is referred to as an "amnesty" or a legalization program because it provides LPR status to aliens who are otherwise residing illegally in the United States. Among IRCA's main provisions was a time-limited legalization program, codified at Section 245A of the Immigration and Nationality Act, that enabled certain illegal aliens who entered the United States before January 1, 1982, to become LPRs. It also had a provision that permitted aliens working illegally as "special agricultural workers" to become LPRs. Nearly 2.7 million aliens established legal status through the provisions of IRCA. How to address the estimated 11.7 million unauthorized aliens residing in the United States is perhaps the most controversial issue in the comprehensive immigration reform (CIR) debate. Reducing the population of unauthorized aliens living in the United States is a common goal, but disagreement about how to do so remains. Some proponents of CIR favor large-scale legalization proposals that would enable most unauthorized residents to become LPRs, if they meet specified conditions and terms as well as pay penalty fees. Some proposals to provide immigration relief to unauthorized aliens would target only particular subsets of this population, such as those with pathways to LPR status under current law (e.g., those with U.S. citizen or LPR family members or employers who have petitioned for them), or unauthorized aliens who were brought to the United States as children. Others counter that Congress should not consider immigration relief or legalization until current laws are more reliably enforced. They also maintain that it would be unfair to reward unauthorized migrants at the expense of potential immigrants who are waiting to come legally and that legalization would serve as a magnet for future unauthorized migrant flows. This CRS report presents data estimating since 1986 the number of unauthorized aliens who have been living in the United States. There have been a variety of estimates of the unauthorized resident alien population over this period, sometimes with substantially different results. This report is limited to analyses of the Current Population Survey (CPS) conducted by the U.S. Census Bureau and the Bureau of Labor Statistics and of the American Community Survey (ACS) conducted by the U.S. Census Bureau so that there are basic standards of comparison over time. Because the CPS and the ACS are both sample surveys of the U.S. population, the results are estimates. Additionally, while the data distinguish between the foreign born who have naturalized and those who have not, they do not identify immigration status (e.g., legal permanent resident, refugee, temporary foreign worker, foreign student, unauthorized alien). Summaries of the detailed analyses of the March CPS, the ACS, and the monthly CPS are presented separately because each of these surveys is based on different questions and sample sizes. The most recent estimate (published in 2013) is that 11.7 million foreign nationals resided in the United States without authorization in 2012. For a basis of comparison, Figure 1 presents the estimate of 3.2 million unauthorized resident aliens in 1986 calculated by demographers Karen Woodrow and Jeffrey Passel, who worked for the U.S. Census Bureau at that time. As expected after the passage of IRCA, the estimate for 1988 dropped to 1.9 million. During the first decade after IRCA, researchers projected that the net growth in unauthorized aliens had averaged about 500,000 annually; analyses done during the early 2000s estimated the average growth at 700,000 to 800,000 unauthorized alien residents annually. By 2002, the estimated number of unauthorized resident aliens had risen to 9.3 million. Appendix B provides further discussion. The estimated number of unauthorized alien residents peaked in 2007, when Passel estimated that there were 12.4 million unauthorized alien residents in the United States ( Figure 1 ). The confidence intervals around the 12.4 million estimate ranged from a low of 11.9 million to a high of 12.9 million. "Thus, average annual growth over the 5-year period since 2000 was more than 500,000 per year. This number reflects the number of new unauthorized migrants arriving minus those who either die, return to their country of origin, or gain legal status." Michael Hoefer, Nancy Rytina, and Bryan C. Baker of the Department of Homeland Security's (DHS's) Office of Immigration Statistics (OIS) published their 2007 estimates of the unauthorized resident alien population from the American Community Survey (ACS) of the U.S. Census Bureau and yielded results consistent with Passel's estimates. An estimated 11.8 million unauthorized immigrants were living in the United States in January 2007 compared to 8.5 million in 2000 (Hoefer, Rytina, and Campbell, 2006). Between 2000 and 2007, the unauthorized population increased 3.3 million; the annual average increase during this period was 470,000. Nearly 4.2 million (35 percent) of the total 11.8 million unauthorized residents in 2007 had entered in 2000 or later. An estimated 7.0 million (59 percent) were from Mexico. The OIS reported an estimated 11.6 million unauthorized alien residents as of January 2008. This estimate marked the first decrease since the OIS began producing annual estimates in 2005. "The decrease in the size of the unauthorized population between 2007 and 2008," the OIS demographers pointed out, "may be due to sampling error in the estimate of the foreign-born population in the 2007 ACS." They estimated that the unauthorized alien population in the United States increased by 37% over the previous eight years. Drawing on a different release of the CPS and a slightly different methodology, Steven Camarota and Karen Jensenius of the Center for Immigration Studies estimated that the unauthorized resident alien population fell to 10.8 million in February 2009. They used tabulations from the monthly CPS data rather than the March supplement to calculate a decrease of 1.7 million (13.7%) in 2009 from their high of 12.5 million unauthorized resident aliens in 2007. Despite the differences in methodology and data, these conclusions were comparable to Hoefer, Rytina, and Baker and to Passel and D'Vera Cohn. Subsequently, Camarota used a combined two-year sample of the CPS (March 2010 and 2011) to estimate 10.5 million unauthorized resident aliens. He pointed out: It must also be remembered that these figures are only for those in the CPS, not those missed by the survey. Estimates prepared by other researchers often adjust for undercount in Census Bureau data. While there is debate about the number missed, most research indicates that roughly 10 percent of illegals are not counted in Census Bureau surveys such as the CPS. Thus, the true size of the illegal population could be 11.5 million. By 2010, most published estimates showed a continued decline in the number of the unauthorized alien residents. Analyses of both the CPS and the ACS data indicated that the estimated number dropped in 2008 and in 2009 before leveling off in 2010. According to Passel and Cohn, "The decline in the population of unauthorized immigrants from its peak in 2007 appears due mainly to a decrease in the number from Mexico, which went down to 6.5 million in 2010 from 7 million in 2007." In 2012, OIS updated their initial 2010 unauthorized estimates, which used Census 2000 population weights updated for births, deaths, and internal and international migration, from 10.8 million to 11.6 million. The revised 2010 estimates are only for the total unauthorized population. OIS indicated that they made other changes, most notably improving the 2011 estimates of the legally resident foreign-born population with the addition of approximately 100,000 LPRs who were not originally included in the annual LPR flow data between 2001 and 2010 because of late or delayed record keying. Hoefer, Rytina, and Baker wrote, "(C)onsidering that the revised 2010 estimate—which is based on more recent data—is higher than the reported estimate, it is possible that the DHS unauthorized estimates for 2005-2009 also underestimate the unauthorized immigrant population." Despite their upward revision in estimates, they concluded: "it is unlikely that the unauthorized immigrant population increased thereafter given relatively high U.S. unemployment, improved economic conditions in Mexico, record low numbers of apprehensions of unauthorized immigrants at U.S. borders, and greater levels of border enforcement." Similarly, Passel, Cohn, and Ana Gonzalez-Barrera of the Pew Research Center revised their estimates in 2013 to conform to the 2010 census. Rather than showing a decrease of 1.3 million from 2007 to 2009, the new estimates show a decrease of 0.1 million from 2007 to 2009. Their new estimates indicate that the decline in authorized resident aliens after 2007 has stalled in 2012, and they suggest the trend may be reversing. Appendix A presents a table detailing their revised estimates. The annual March CPS provides detailed socioeconomic data that are not available in the monthly CPS, shedding further light on the traits and trends in authorized migration. The most recent detailed analysis that Passel and Cohn have done is for 2010. As Figure 2 illustrates, the 2010 distribution of the unauthorized population by region of origin was similar to Woodrow and Passel's analysis of the 1986 data, despite the growth in overall numbers from 3.2 million in 1986 to 11.2 million in 2010. There were 2.6 million, or 23%, from Latin American countries (excluding Mexico) in 2010. Asia's share of the unauthorized alien residents appeared to have grown over this period (from 6% to 11%), and the estimated numbers of unauthorized resident aliens from Asia rose from 0.2 million in 1986 to 1.3 million in 2010. In 1986, 69% of the unauthorized aliens residing in the United States were estimated to be from Mexico, compared with 58% in 2010. While the sheer number of unauthorized Mexicans residing in the United States increased substantially from 2.1 million in 1986 to a peak of almost 7 million in 2007, the estimated number fell to 6.1 million in 2011. "The sharp downward trend in net migration from Mexico began about five years ago," writes Passel, Cohn, and Gonzalez-Barrera, "and has led to the first significant decrease in at least two decades in the unauthorized Mexican population." Passel and Cohn's analysis of the 2010 CPS finds that about one-third of the estimated 11.2 million unauthorized aliens resided in two states: California and Texas, as the map reproduced in Figure 3 shows. Another group of states—Florida, New York, New Jersey, and Illinois—have unauthorized resident alien populations exceeding half a million. Passel and Cohn also observed declines in the number of unauthorized resident aliens. The four individual states where the number of unauthorized immigrants declined from March 2007 to March 2010 were New York, Florida, Virginia and Colorado. Additionally, the combined unauthorized immigrant population in Arizona, Nevada and Utah also decreased during that period, although the change was not statistically significant for any of those states individually. They also found that the combined unauthorized resident alien population of Arizona, Nevada, and Utah decreased from an estimated 850,000 in 2007 to an estimated 700,000 in 2010. As part of an earlier analysis, Passel and Cohn also calculated estimates of the number of households with unauthorized aliens. They reported that the number of persons living in families in which the head of the household or the spouse was an unauthorized alien was an estimated 16.6 million in 2008. They also reported that there were an estimated 1.5 million unauthorized children and an estimated 4.0 million citizen children who were living in families in which the head of the family or a spouse was unauthorized in 2008. The American Community Survey (ACS) is a national sample survey that consists of non-overlapping samples from which the U.S. Census Bureau collects monthly household data over the course of a year. It was tested in 2000 and fully implemented in 2005. Although it does not enable the post-1986 time series analysis that the CPS offers, it has a larger sample size. As a result, OIS demographers Hoefer, Rytina, and Baker decided: "The ACS was selected for the estimates because of its large sample size, about 3 million households in 2007 compared to 100,000 for the March 2008 Current Population Survey, the primary alternative source of national data on the foreign-born population." They drew on the larger sample size of the ACS to analyze changes in the unauthorized resident alien population between 2000 and 2010. As has been the case for many years, California continued to be the top state of residence of the unauthorized population in 2011, with an estimated 2.8 million ( Figure 4 ). However, California's share of the total unauthorized population declined from 30% in 2000 to 25% in 2011, according to the OIS demographers. While the map in Figure 3 puts Texas on a par with California, the OIS analysis of the ACS data estimates that Texas has 1 million less than California—1.8 million unauthorized residents. As Figure 4 depicts, Florida had 740,000, ranking it third in unauthorized alien residents. Florida, however, was the only state among the top 10 to have a decline in the estimated number of unauthorized alien residents, an 8% decrease from 0.8 million to 0.74 million. Among the 10 leading states of residence of the unauthorized population in 2011, OIS reported that those with the greatest percentage increases in unauthorized aliens from 2000 to 2011 were Georgia (95%), Texas (64%), North Carolina (53%) and Washington (51%). Migrants from Mexico continued to dominate the unauthorized alien population in the ACS, as Figure 5 illustrates, and as they had in the CPS data shown in Figure 2 The OIS demographers estimated that the unauthorized resident alien population from Mexico increased from 4.7 million in 2000 to 6.8 million in 2011, an estimate larger than Passel and Cohn's estimate of 6.1 million unauthorized resident alien population from Mexico in 2011. The 10 leading countries of origin represented over 80% of the unauthorized alien population in 2011 and are presented in Figure 5 . The other top source countries were El Salvador, Guatemala, Honduras, the Philippines, India, Ecuador, Vietnam, Korea, and China. Asian nations made up five of the top 10 source countries for unauthorized resident aliens in 2011. In terms of the percentage change from 2000 to 2011, Honduras led among the top 10 source countries with a 132% increase. India's percentage change from 2000 to 2011 was 94%, and Ecuador and Guatemala were 83% and 82%, respectively. Interestingly, Vietnam became a top 10 source country as much because of the decrease among other source countries (from 1.94 million in 2000 to 1.75 million in 2011) as its increase from 0.16 million in 2000 to 0.17 million in 2011. Although the stereotypic image of an unauthorized alien is a young male, females have made up a sizeable minority of the unauthorized alien population for many years. The OIS demographers estimated over 5.3 million females among the unauthorized alien population in 2011. Males comprised the majority—an estimated 6.1 million or 53%—of the unauthorized alien population in 2011. As Figure 6 illustrates, unauthorized resident males tended to cluster in the peak working age groups of 18-24, 25-34, and 35-44. Although not larger in actual numbers, unauthorized resident females are more likely than males to be among those 17 and younger as well as those age 45 and older, according to the OIS demographers. Figure 7 depicts that the portion of the resident unauthorized population that arrived from 2005 to 2010 was estimated to be 14%, the lowest five-year interval since 1980-1984 (7%). The largest five-year interval was 2000-2004, during which an estimated 3.3 million unauthorized alien residents came to the United States. Reportedly, unauthorized aliens from Mexico arrived at an average of 500,000 annually during this period. A nearly comparable number of unauthorized residents—3 million—arrived during the prior five-year period (1995-1999). Only 1.6 million unauthorized resident aliens came to the United States during 2005-2010. These data further suggest that the rate of unauthorized migration slowed in recent years. Steven Camarota and Karen Jensenius of the Center for Immigration Studies used the monthly CPS data to estimate that the unauthorized resident alien population has fallen from 12.5 million in 2007 to 10.8 million in 2009. Their analysis of the monthly data from January 2000 through January 2009 found that their estimate of the unauthorized alien population generally tends to be larger in the spring and summer, when employment in agriculture, hospitality, and construction tends to rise, although this pattern was not evidenced every year. The monthly CPS provides more timely data, but offers much more limited demographic, immigration, and citizen information than the annual March CPS Supplement or the annual ACS. In the absence of more detailed immigration data, Camarota and Jensenius opted to use Hispanics aged 18 to 40 who have no more than a high school diploma for what they call "likely illegal population" or their proxy for unauthorized migration. They then offered the following observation: [S]ince the peak in July 2007, the likely illegal Mexican population has declined 13.4 percent through February of this year…. On average, since January of 2008 the likely illegal population is about 7 percent lower each month compared to same month in the prior year. This is true through the first quarter of 2009. As already discussed, if we compare the two-year period of the first quarter of 2007 (which is reported as February) to the first quarter of 2009 we find a total decline of 10.9 percent in the likely illegal population. They posited that "the observed decline must be due to a combination of less-educated young Hispanic immigrants leaving the country and fewer entering the country." Acknowledging that their analysis was over simplified and not inclusive of other factors likely to reduce unauthorized migration, Camarota and Jensenius estimated that out-migration of those Mexicans aged 18-40 who have no more than a high school diploma might have been more than twice as large in the 2006 to 2009 period as in the 2002 to 2005 period. The research points to various factors that have contributed to the increase in unauthorized resident aliens over the past two decades as well as a leveling off of these trends in recent years. Historically, unauthorized migration is generally attributed to the "push-pull" of prosperity-fueled job opportunities in the United States in contrast to limited or nonexistent job opportunities in the sending countries. Accordingly, the economic recession that began in December 2007 may have curbed the migration of unauthorized aliens, particularly because sectors that traditionally rely on unauthorized aliens, such as construction, services, and hospitality, have been especially hard hit. Some researchers maintain that lax enforcement of employer sanctions for hiring unauthorized aliens facilitated the "pull" for many years and that the ratcheting up of work site enforcement in 2007 and 2008 has subsequently mitigated the flow. Trend data suggest a correlation, but it remains difficult to demonstrate this element empirically, especially because the increased worksite enforcement and removals were coincident with the housing downturn and the onset of the economic recession. Political instability or civil unrest at home is another element that traditionally has induced people to risk unauthorized migration. Asylum seekers who enter the United States illegally have always been included in the estimates of the unauthorized alien population. Asylum claims ebbed and flowed in the 1980s and peaked in FY1996. Since FY1997, defensive asylum claims have declined overall, dropping by 53% through FY2009 and rising slightly since 2010. Comparatively, asylum seekers have become a smaller share of the unauthorized resident alien population and do not account for the overall trends in the unauthorized resident aliens in recent years. Although most policy makers have assumed that tighter border enforcement reduces unauthorized migration, some researchers have observed that the strengthening of the immigration enforcement provisions, most notably by the enactment of the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 (IIRIRA), may have inadvertently increased the population of unauthorized resident aliens. This interpretation, generally referred to as a caging effect, argues that IIRIRA's increased penalties for illegal entry, coupled with increased resources for border enforcement particularly after the September 11, 2001, terrorist attacks, stymied what had been a rather fluid movement of migratory workers along the southern border; this in turn raised the stakes in crossing the border illegally and created an incentive for those who succeed in entering the United States to stay. More recently, some maintain that strengthened border security measures, such as "enforcement with consequences" and coordinated efforts with Mexico to reduce illegal migrant recidivism may be having an impact. The increased number of U.S. Border Patrol agents from 12,349 agents in FY2006 to 21,444 agents in FY2012 illustrates a clear inverse relationship with the leveling off of unauthorized migration over the same period. However, these border patrol resource trends also correlate with unemployment levels over this period. While the empirical evidence to support a causal link between strengthened border enforcement policies and reduced illegal migration has not been demonstrated, some researchers credit it as part of a constellation of factors holding down the flow. The current system of legal immigration is frequently cited as another factor that contributed to the growth in unauthorized alien residents. There are statutory ceilings that limit the type and number of immigrant visas issued each year, which lead to wait-times for visas to become available to legally come to the United States. Some observe that family members sometimes risk staying in the United States on an expired temporary visa or entering the United States illegally to be with their family while they wait for the visas to become available. It remains difficult, however, to demonstrate a causal link or to guarantee that increased levels of legal migration would absorb the current flow of unauthorized migrants. The increase in the number of aliens deported from the United States annually from 189,026 in 2001 to 387,242 in 2010 might also have had a chilling effect on family members weighing illegal presence. Some observers point to more elusive factors—such as pronounced shifts in immigration enforcement priorities away from illegal entry to identifying and removing suspected terrorists and criminal aliens, or well-publicized legislative debates of possible "amnesty" legislation—as having magnet effects when they assess the increase in unauthorized resident aliens over the past 25 years. It is difficult to measure whether, or to what extent, these other phenomena have contributed to the flow of unauthorized resident aliens. Appendix A. Pew Research Center's 2013 Estimates The table below is taken directly from Population Decline of Unauthorized Immigrants Stalls, May Have Reversed by Passel, Cohn, and Gonzalez-Barrera, in which they revised their estimates to conform to the 2010 census. They concluded: "The resulting series of unauthorized immigrant estimates are consistent over time but inconsistent with previous estimates." Their new estimates have unauthorized levels peaking at 12.2 million in 2007 rather than 12.4 million. They also have revised estimates for 2009, from 11.1 million to 11.3 million, among other revisions to earlier estimates. In other words, the revisions have smoothed out the recent trends. Appendix B. Early Estimates of the Unauthorized Resident Alien Population According to demographer Robert Warren of the former Immigration and Naturalization Service (INS), the estimated unauthorized resident alien population grew to 3.4 million in 1992 and to 5.0 million in 1996. By the close of the decade, the estimated number of unauthorized alien residents had more than doubled. Jeffrey Passel, now at the Pew Research Center, estimated the unauthorized population in 2000 at 8.5 million, but this latter estimate included aliens who had petitions pending or temporary relief from deportation. Subsequently, Warren estimated that there were 7.0 million unauthorized aliens residing in the United States in 2000. Warren later revised his earlier analyses using the latest CPS and estimated that there were 3.5 million unauthorized aliens living in the United States in 1990 and 5.8 million in 1996. Unlike Passel, Warren excluded "quasi-legal" aliens (e.g., those who had petitions pending or relief from deportation) from his estimates. 

Summary: Estimates derived from the March Supplement of the U.S. Census Bureau's Current Population Survey (CPS) indicate that the unauthorized resident alien population (commonly referred to as illegal aliens) rose from 3.2 million in 1986 to 12.4 million in 2007, before leveling off at 11.7 million in 2012. The estimated number of unauthorized aliens had dropped to 1.9 million in 1988 following passage of a 1986 law that legalized several million unauthorized aliens. Jeffrey Passel, a demographer with the Pew Research Center, has been involved in making these estimations since he worked at the U.S. Bureau of the Census in the 1980s. Similarly, the Department of Homeland Security's Office of Immigration Statistics (OIS) reported an estimated 11.5 million unauthorized alien residents as of January 2011, up from 8.5 million in January 2000. The OIS estimated that the unauthorized resident alien population in the United States increased by 37% over the period 2000 to 2008, before leveling off since 2009. The OIS estimated that 6.8 million of the unauthorized alien residents in 2011 were from Mexico. About 33% of unauthorized residents in 2011 were estimated to have entered the United States since 2000, but the rate of illegal entry appears to be slowing. The OIS based its estimates on data from the U.S. Census Bureau's American Community Survey. Although increased border security, a record number of alien removals, and high unemployment, among other factors, have depressed the levels of illegal migration in recent years, the number of unauthorized aliens residing in the United States remains sizeable. Research suggests that various factors have contributed to the ebb and flow of unauthorized resident aliens, and that the increase is often attributed to the "push-pull" of prosperity-fueled job opportunities in the United States in contrast to limited job opportunities in the sending countries. Accordingly, the economic recession that began in December 2007 may have curbed the migration of unauthorized aliens, particularly because sectors that traditionally rely on unauthorized aliens, such as construction, services, and hospitality, have been especially hard hit. Some researchers also suggest that the increased size of the unauthorized resident population during the late 1990s and early 2000s is an inadvertent consequence of border enforcement and immigration control policies. They posit that strengthened border security curbed the fluid movement of seasonal workers. This interpretation, generally referred to as a caging effect, argues that these policies raised the stakes in crossing the border illegally and created an incentive for those who succeed in entering the United States to stay. More recently, some maintain that strengthened border security measures, such as "enforcement with consequences," coordinated efforts with Mexico to reduce illegal migrant recidivism, and increased border patrol agents, may be part of a constellation of factors holding down the flow. The current system of legal immigration is cited as another factor contributing to unauthorized migration. The statutory ceilings that limit the type and number of immigrant visas issued each year create long waits for visas. According to this interpretation, many foreign nationals who have family in the United States resort to illegal avenues in frustration over the delays. Some researchers speculate that record number of alien removals (e.g., reaching almost 400,000 annually since FY2009) caused a chilling effect on family members weighing unauthorized residence. Some observers point to more elusive factors when assessing the ebb and flow of unauthorized resident aliens—such as shifts in immigration enforcement priorities away from illegal entry to removing suspected terrorists and criminal aliens, or well-publicized discussions of possible "amnesty" legislation.