Unauthorized Aliens Residing in the United States: Estimates Since 1986

Ruth Ellen Wasem
Specialist in Immigration Policy

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 11.2 million in 2010. Jeffrey Passel, a demographer with the Pew Hispanic Research Center, has been involved in making these estimations since he worked at the U.S. Bureau of the Census in the 1980s. 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. The estimates of unauthorized aliens peaked at an estimated 12.4 million in 2007. About 39% of unauthorized alien residents in 2010 were estimated to have entered the United States in 2000 or later.

Similarly, the Department of Homeland Security’s Office of Immigration Statistics (OIS) reported an estimated 10.8 million unauthorized alien residents as of January 2010, up from 8.5 million in January 2000. The OIS estimated that 6.6 million of the unauthorized alien residents were from Mexico, an estimate comparable to Passel and D’Vera Cohn’s calculation of 6.5 million. The OIS based its estimates on data from the U.S. Census Bureau’s American Community Survey. The OIS estimated that the unauthorized resident alien population in the United States increased by 37% over the period 2000 to 2008, then leveled off in 2009 and 2010.

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 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 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 has curbed the fluid movement of seasonal workers. This interpretation, generally referred to as a caging effect, argues that these policies have raised the stakes in crossing the border illegally and created an incentive for those who succeed in entering the United States to stay.

The current system of legal immigration is cited as another factor contributing to unauthorized alien residents. 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 would prefer to come to the United States legally resort to illegal avenues in frustration over the delays. It is difficult, however, to demonstrate a causal link or to guarantee that increased levels of legal migration would absorb the current flow of unauthorized migrants. Furthermore, some researchers speculate that the doubling in deportations since 2001 might also have had a chilling effect on family members weighing unauthorized residence in recent years.

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 discussions of possible “amnesty” legislation. This report does not track legislation and will be updated as needed.


Date of Report: September 22, 2011
Number of Pages: 17
Order Number: RL33874
Price: $29.95

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