Order Code RS21938
Updated January 24, 2007
Unauthorized Aliens in the United States:
Estimates Since 1986
Ruth Ellen Wasem
Specialist in Immigration Policy
Domestic Social Policy Division
Estimates derived from the March Supplement of the U.S. Census Bureau’s Current
Population Survey indicate that the unauthorized resident alien population (commonly
referred to as illegal aliens) has risen from 3.2 million in 1986 to 10.3 million in 2004.
The estimated number of unauthorized aliens had dropped to 1.9 million in 1988
following passage of a law that legalized many unauthorized aliens. Research suggests
that a constellation of factors has contributed to the increase in unauthorized resident
aliens, including the “push-pull” of a prosperous economy, the inadvertent consequence
of border enforcement policies that have curbed the fluid movement of migrant workers,
and the backlogs in processing immigrant petitions. Some observers also assert that
resources for enforcement of immigration laws in the interior of the country are
inadequate. This report does not track legislation and will be updated as needed.
An estimated 35.7 million foreign-born people resided in the United States in 2004.
In recent years, the United States typically admits or adjusts 600,000 to 1 million aliens
annually, giving them the status of “legal permanent resident” (LPR), which is commonly
known as an immigrant. In addition to those foreign nationals who permanently reside
legally in the United States, millions each year come temporarily on nonimmigrant visas,
and some of these nonimmigrants (e.g., foreign students and intra-company business
transfers) may reside legally in the United States for several years. It is also estimated that
each year hundreds of thousands of foreign nationals overstay their nonimmigrant visas
or enter the country illegally and thus are unauthorized aliens.
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 § 245A of the Immigration and Nationality Act, that
enabled certain illegal aliens who entered the United States before January 1, 1982, to
become LPRs.1 It also had a provision that permitted aliens working illegally as “special
agricultural workers” to become LPRs.2 Nearly 2.7 million aliens established legal status
through the provisions of IRCA.
Continued high levels of unauthorized migration to the United States have, in part,
prompted the current discussion of guest worker programs, and several major legislative
proposals include provisions that would permit legalization under specified conditions.3
There are also legislative proposals aimed at reducing unauthorized migration by
tightening up enforcement of immigration laws.4 The report of the National Commission
on Terrorist Attacks Upon the United States (also known as the 9/11 Commission) stated
that “... more than 9 million people are in the United States outside the legal immigration
system” as one of the reasons for the Commission’s recommendations to improve
immigration services and strengthen enforcement of immigration laws.5
This report presents data estimating the number of unauthorized aliens who have
been living in the United States since 1986. 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 data analyses of the Current Population Survey
(CPS) conducted by the U.S. Census Bureau and the Bureau of Labor Statistics so that
there is a basic standard of comparison over time.6
8 U.S.C. § 1255a.
8 U.S.C. § 1160.
For a discussion of current legalization proposals, see CRS Report RL32044, Immigration:
Policy Considerations Related to Guest Worker Programs, by Andorra Bruno; CRS Report
RL33125, Immigration Legislation and Issues in the 109th Congress, coordinated by Andorra
Bruno; and CRS Report RL31365, Unauthorized Alien Students: Issues and Legislation, by
Andorra Bruno and Jeff Kuenzi.
For further discussions of immigration enforcement legislation, see CRS Report RL32369,
Immigration-Related Detention: Current Legislative Issues, by Alison Siskin; CRS Report
RL32270, Enforcing Immigration Law: The Role of State and Local Law Enforcement, by Lisa
M. Seghetti and Stephen Viña; and CRS Report RL33125, Immigration Legislation and Issues
in the 109th Congress, coordinated by Andorra Bruno.
For a discussion of these recommendations, see National Commission on Terrorist Attacks
Upon the United States, The 9/11 Commission Report, chap. 12.4, pp. 383-391, July 2004.
The demographers who conducted these analyses all used some variant of a residual
methodology to estimate the population (i.e., the estimated population remaining after citizens
and authorized aliens are accounted for), another reason they were selected for this comparison.
Demographers at the U.S. Census Bureau also have used a similar methodology to estimate the
residual foreign born population in the 2000 decennial census, and they reported the following.
“According to our calculations, the estimated residual foreign-born population counted in the
2000 census was 8,705,419. Assuming a 15-percent undercount rate yields a population of
10,241,669 in 2000.” They point out that the category of residual foreign born includes “quasi
legal aliens” (i.e., aliens without legal status who have petitions pending or court cases underway
that potentially would give them LPR status) as well as unauthorized aliens and thus should not
be considered an official estimate of unauthorized resident aliens. U.S. Census Bureau,
Population Division Working Paper 61, Evaluating Components of International Migration: The
Residual Foreign Born, by Joseph M. Costanzo, Cynthia Davis, Caribert Irazi, Daniel Goodkind,
Estimates Since 1986
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.7 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
1996.8 By the close of the decade, the estimated number of unauthorized alien residents
had more than doubled. Passel, now at the Pew Hispanic Center, estimated the
unauthorized population in 2000 at 8.5 million, but this latter estimate included aliens
who had petitions pending or relief from deportation.9
Figure 1. Estimates of the Unauthorized Resident Alien
Source: CRS presentation of analysis of Current Population Survey data conducted by Karen Woodrow
and Jeffrey Passel (1986 and 1988), Robert Warren (1990, 1992, 1996, and 2000), Jeffrey Passel (2000),
Jeffrey Passel, Randy Capps and Michael Fix (2004), and Passel (2005).
and Roberto Ramirez. June 2002.
Karen Woodrow and Jeffrey Passel, “Post-IRCA Undocumented Immigration to the United
States: An Analysis Based on the June 1988 CPS,” in Undocumented Migration to the United
States, by Frank D. Bean, Barry Edmonston and Jeffrey Passel (RAND Corporation, 1990).
Annual Estimates of the Unauthorized Immigrant Population Residing in the United States and
Components of Change: 1987 to 1997, by Robert Warren, Office of Policy and Planning, U.S.
Immigration and Naturalization Service, Sept. 2000.
U.S. Congress, House Committee on the Judiciary, Subcommittee on Immigration and Claims,
Hearing on the U.S. Population and Immigration, Aug. 2, 2001.
Subsequently, Warren estimated that there were 7.0 million unauthorized aliens
residing in the United States in 2000. As depicted in Figure 1, he also 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. Warren excluded
“quasi-legal” aliens (e.g., those who had petitions pending or relief from deportation)
from his estimates.10 By 2002, the estimated number of unauthorized resident aliens had
risen to 9.3 million.11 While the net growth in unauthorized aliens had averaged about
500,000 annually, recent analyses estimate the average growth at 700,000 to 800,000
annually. If the later trend holds, about 12 million unauthorized aliens may be residing
in the United States in 2006.
Unauthorized Alien Residents in 200412
The most recent published estimate based upon the March 2004 CPS is that 10.3
million unauthorized aliens are residing in the United States. According to this analysis
by Passel, Mexicans make up over half of undocumented immigrants — 57 % of the total,
or about 5.9 million. He estimates that 2.5 million (23%) are from other Latin American
countries. About 9% are from Asia, 6% from Europe and Canada, and 4% from the rest
of the world. Passel estimates the number of persons living in families in which the head
of the household or the spouse is an unauthorized migrant is 13.9 million as of March
As Figure 2 illustrates, the 2004 distribution by region of origin is similar to
Woodrow and Passel’s analysis of the 1986 data, despite the growth in overall numbers
from 3.2 million in 1986 to 10.3 million in 2004. In 1986, 69% of the unauthorized aliens
residing in the United States were estimated to be from Mexico compared to 57% in 2004.
Asia’s share of the unauthorized alien residents appears to have grown over this period
(from 6% to 9%), as has the portion from the “other” parts of the world. Note that Canada
is grouped with North and South America (excluding Mexico) in 1986 and with Europe
In terms of the distribution of unauthorized aliens across the country in 2004, Passel,
calculates that almost two-thirds (68 %) live in eight states: California (24%), Texas
(14%), Florida (9%), New York (7%), Arizona (5%), Illinois (4%), New Jersey (4%), and
North Carolina (3%). “According to estimates for 1990,” Passel writes, “the
undocumented population lived in only six states that had been traditional settlement
areas for the foreign-born — California. New York, Texas, Illinois, Florida and New
Jersey.” Passel observes, however, that the most rapid growth in the unauthorized resident
aliens since the mid-1990s has been outside these six states.
U.S. Immigration and Naturalization Service, Estimates of the Unauthorized Immigrant
Population Residing in the United States, 1990 to 2000, Jan. 31, 2003.
The Urban Institute, Undocumented Immigrants: Facts and Figures, by Jeffrey Passel, Randy
Capps, and Michael Fix, Jan. 12, 2004.
Pew Hispanic Center, Estimates of the Size and Characteristics of the Undocumented
Population, by Jeffrey Passel, March 21, 2005.
Figure 2. Unauthorized Resident Alien Population, 1986 and 2004
North & South America
Other Latin America
Source: CRS presentation of analysis of Current Population Survey data conducted by Karen Woodrow and
Jeffrey Passel (1986 and 1988), and Jeffrey Passel (2005).
The research points to a constellation of factors that have contributed to the increase
in unauthorized resident aliens. 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.13 Some
observers maintain that lax enforcement of employer sanctions for hiring unauthorized
aliens has facilitated this “push-pull,” but it is difficult to empirically demonstrate this
element. Political instability or civil unrest at home is another element that traditionally
has induced people to risk unauthorized migration, but the motives for such migrations
are sometimes mixed with the economic hardships that are often correlated with political
For a discussion of how many unauthorized aliens are currently in the U.S. workforce, see CRS
Report RL32044, Immigration: Policy Considerations Related to Guest Worker Programs, by
Andorra Bruno, pp. 6-7. For trends in apprehensions of unauthorized aliens, see CRS Report
RL32562, Border Security: The Role of the U.S. Border Patrol, by Blas Nuñez-Neto.
For a summary of this research, see Commission for the Study of International Migration and
Cooperative Economic Development, Unauthorized Migration: An Economic Development
Response, Appendix E, July 1990.
Although most policy makers have assumed that tighter border enforcement would
reduce unauthorized migration, some researchers are now suggesting 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
perspective argues that IIRIRA’s increased penalties for illegal entry coupled with
increased resources for border enforcement 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.15
Another contributing factor — best represented by the “quasi-legal” aliens discussed
above — is the wait-times for immigrant petitions to be processed and visas to become
available to legally come to the United States. There are statutory ceilings that limit the
number of immigrant visas issued each year. There are also significant backlogs in
processing petitions due to the high volume of aliens eligible to immigrate to the United
States and the large number eligible to become U.S. citizens. Of the pending cases,
reportedly almost 2 million are immediate relative and family preference petitions.16
Many observe that these family members sometimes risk residing without legal status
with their family in the United States while they wait for the petitions to be processed or
visas to become available.
Some observers point to more elusive factors — such as shifts in immigration
enforcement priorities away from illegal entry to removing suspected terrorists and
criminal aliens or discussions of possible “amnesty” legislation — when they assess the
increase of unauthorized resident aliens. Others argue that border security measures
enacted in recent years have not received adequate funding to be effective against
unauthorized migration, and some maintain that state and local law enforcement officers
have not been sufficiently involved in apprehending illegal aliens. Some would make
illegal presence an aggravated felony.17 Still others assert that there has not been
sufficient funding and staffing for enforcement of immigration laws in the interior of the
country.18 It is difficult to measure whether, or to what extent, these other phenomena
have contributed to the increase in unauthorized resident aliens.
For analysis of the IIRIRA’s effect on unauthorized alien residents, see Wayne Cornelius,
“Death at the Border: Efficacy and Unintended Consequences of U.S. Immigration Control
Policy,” Population and Development Review, vol. 27, no.4 (Dec. 2001). For an analysis of the
reduction in unauthorized alien apprehensions after IRCA, see Thomas J. Espenshade,
“Undocumented Migration to the United States: Evidence from a Repeated Trials Model,” in
Undocumented Migration to the United States, by Frank D. Bean, Barry Edmonston and Jeffrey
Passel (RAND Corporation, 1990).
For analysis of immigration admissions, visa priority dates, and backlogs, see CRS Report
RL32235, U.S. Immigration Policy on Permanent Admissions, by Ruth Ellen Wasem.
For a full discussion of these legal and policy issues as well as current legislation, see CRS
Report RL33125, Immigration Legislation and Issues in the 109th Congress, coordinated by
Andorra Bruno; and CRS Report RL33181, Immigration Related Border Security Legislation in
the 109th Congress, by Blas Nuñez-Neto.
For a summary of recent funding, see CRS Report RL33049, FY2006 Appropriations for
Border and Transportation Security, coordinated by Jennifer E. Lake and Blas Nuñez-Neto.