The actual number of unauthorized aliens in the United States is unknown. As expected after the passage of IRCA, the estimate for 1988 dropped to 1.9 million. 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 ). 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. The OIS reported an estimated 11.6 million unauthorized alien residents as of January 2008. Thus, the true size of the illegal population could be 11.5 million. 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. 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. These data further suggest that the rate of unauthorized migration slowed 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. 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. 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. 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.
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.