January 1980 (Rev.)
Preliminary Review of Existing Studies of the Number of
Illegal Residents in the United States*
Jacob S. Siegel
Jeffrey S. Passel
J. Gregory Robinson
U.S. Bureau of the Census
Despite a paucity of data and the obvious difficulties of the task, several
attempts have been made to estimate the number of illegal residents in the United
States for various dates in the 1970's (table 1). The methods employed to derive
the estimates have varied widely. Some of the estimates are speculative and others
are empirically based. Figures offered by former Commissioners Farrell and
Chapman, and Associate Commissioner Guss, of the Immigration and Naturalization
Service (INS) are essentially speculative. This description may also be applied
to the estimates based on the use of the Delphi method offered by Lesko Associates
in 1975. However, several studies are now available which employ analytical
techniques, either statistical analysis (Lancaster and Scheuren, 1978; Heer, 1979;
Robinson, 1979; Mexico, CENIET, 1979a) or survey methods (Mexico, CENIET, 1979b).
In the following sections, the various estimates will be reviewed and assessed.
A further note is offered on what is known about the geographic distribution of
the resident illegal population.
1/ The term "illegal resident" is used here to denote persons who would be
considered usual residents of the United States for purposes of census enumeration
but who are deportable because they violated the statutes regarding entry to the
United States or because they violated the terms of their admission after being
admitted legally. The term encompasses, therefore, those who "entered without
inspection," "visa abusers" or "overstayers," and "fraudulent entrants." These
groups as a whole have been described variously by the terms "illegal aliens,"
"illegal migrants," "undocumented workers," "deportable aliens," etc.
2/ For another recent review of the studies on illegal immigration, see Morris
and Mayio (1980 forthcoming); earlier reviews include U.S. Department of Justice
(1976b) and Keely (1977).
*This paper was prepared as a working document at the request of and for the use of
the Research Staff of the Select Commission on Immigration and Refugee Policy.
This review of the existing studies on illegal residents finds that there are
currently no reliable estimates of the number of illegal residents in the United
States or of the net volume of illegal immigration to the United States in any
recent past period. Although even the approximate number of illegal residents
remains uncertain, inferences can be drawn with regard to the rough magnitude of
the number. At the end of this review, the authors offer their own cautious
INS Speculative and Conjectural Estimates
Several conjectural estimates of the number of illegal residents living in the
United States were made during the 1970's by officials of the Immigration and
Naturalization Service. Commissioner Farrell announced an estimate of 1 million
illegal residents for 1972 which was the result of a series of "educated guesses."
The number of aliens in the categories of overstays, crewmen deserters, and
stowaways was guessed to be 175,000 for 1972. In 1971, these categories accounted
for 19 percent of the apprehensions and entries without inspection accounted for
81 percent. These percentages were then assumed to apply to the illegal population
at large in 1972.
A description of the "methodology" underlying Commissioner Chapman's estimates
of 4 to 12 million for 1975 has never been made available by the INS. Since
presumably it lacked any form of empirical basis, it must be viewed as arbitrary
In October 1975, Lesko Associates released a report, prepared under contract
to the Immigration and Naturalization Service, which contained two separate
estimates of the number of illegal residents in the United States in 1975. The
Lesko estimate of 8.2 million for the total illegally resident population resulted
from an application of the Delphi technique. This technique might be characterized
as iterativee informed guesswork." The Lesko estimate has been widely criticized
in various public forums as a poorly implemented application of this technique
because, among other reasons, important stakeholders were omitted from the panel
and the panelists provided no rationale for their initial estimates or changes
in these estimates. The second Lesko estimate of 5.2 million for Mexican illegal
residents has also been soundly criticized on several counts (Library of Congress
Congressional Research Service, 1976; Roberts et al., 1978). In general, the Lesko
estimates are based on extremely weak and untenable assumptions.
The INS estimate of 6 million for 1976 (Chapman, 1976a; Guss, 1977) was based
on responses to requests to the INS District Directors from INS headquarters for
regional estimates of illegal residents. The district offices were asked to provide,
in addition to estimates of illegals for their districts, a description of the
methodology used to generate the estimates. None gave specific procedures.
Rather, all but one referred to the "experience" of officials as the basis for
the estimate; the other claimed no "scientific" basis at all for his estimate.
Thus, the overall estimate may be characterized as "synthetic speculation."
An estimate of 3 to 6 million illegal residents for 1978 was given by
Commissioner Castillo in Congressional testimony. This estimate is simply a
distillation of previously available estimates.and is not based on any new
research or new data. In his own testimony, Castillo characterized his
estimate as "soft."
INS Record Checks and Field Studies
In two of its own studies, the INS attempted to estimate the number of illegal
residents who entered the United States with fraudulent documents and the number who
overstayed their visas. In a study carried out in 1976, the visa records of
nonimmigrant aliens admitted into the United States and the visa records of persons
subsequently departing were matched so as to identify those who had failed to leave
the country when required (Chapman, 1976b). From 1974 to 1976, 95 percent of the
departure records received were successfully matched with the arrival records. The
other 5 percent represent errors in the records, e.g., persons whose entry form was
not picked up by INS or was lost by INS. The number of overstays was calculated by
subtracting out of total arrivals those that were matched and those that are still
legally entitled to be in the United States. On this basis, INS reported in
mid-1976 that 740,000 nonimmigrant aliens failed to depart since 1974 and that
over a million unresolved departures were left outstanding from before 1974.
An experiment was conducted by INS between September 1975 and February 1976,
during which two teams of immigration inspectors, stationed at selected Mexican
border points (15) and at major airports (10), carefully inspected prospective
alien entrants for fraudulent documents and for evidence of intent to violate visa
terms (U.S. Departmentr:'dJustice, 1976a). Rates of identification of fraudulent
documents by the teams at each entry point were "extrapolated" to produce an estimate
of about 500,000 successful fraudulent entries into the country during 1975. The
reliability of this estimate is questionable considering the extremely small number
of cases used to derive it and the attendant problems of sampling and interview bias.
In recognition of the pressing need for "hard" data on illegal residents,
the INS sponsored a special Residential Survey of the foreign-born population in
12 States in 1978 that was designed to provide an estimate of the number of illegal
residents in the United States and information on their demographic and socio-
economic characteristics. Pretests were conducted in several cities by a
private firm (Reyes Associates and subcontractors) in the fall of 1977; the
full-scale survey was carried out between May and August of 1978. Unfortun-
ately, as a result of cost overruns in carrying out the interviews and
other difficulties, the field work was not completed and the results have not
been made available. It is still possible that the findings of the full-scale
survey will be forthcoming.
Analytic estimates of the number of illegal residents in the United States
began to be undertaken when the "softness" of the INS conjectural estimates became
apparent. The first relates to the year 1970 or the period 1960-70 but was confined
to Mexican illegal residents. An intercensal cohort analysis of Mexican census data
for 1960 and 1970 carried out by Goldberg (1974) resulted in an estimate of net
illegal Mexican immigration to the United States of 1.6 million between 1960 and
1970. Goldberg's method consisted of applying 10-year survival rates to the 1960
census counts for Mexico to estimate the "expected" population in 1970 and then
taking the differences between the census counts in 1970 and the "expected" figures,
after allowing for legal immigration into the United States. The sum over all ages
of the adjusted differences is taken to represent net illegal immigration.
The accuracy of Goldberg's estimate rests on several assumptions that cannot
be substantiated. The critical assumption of his method is that the amount of net
underenumeration of population in the 1960 and 1970 Mexican censuses was the same.
It can be shown that even a small or zero difference in the percent completeness of
coverage between the 1960 and 1970 censuses can greatly affect Goldberg's estimate of
illegal Mexican immigration into the United States. For example, the assumption that
the Mexican census of 1970 had the same 5-percent omission rate as the census of 1960,
implying a far larger amount of underenumeration in 1970 than in 1960, would lower
the estimate from 1.6 million to well below 1 million.
The problem of estimating underenumeration in two censuses can be circumvented
by building up the "expected" population figure for 1970 from birth and death
statistics. The estimation problem then involves deriving accurate estimates of
the completeness of birth and death registration for a number of years as well
as estimates of underenumeration for the 1970 Mexican census. This approach has
been tried in an unpublished paper by Warren (1979); his results are inconclusive,
The Lancaster and Scheuren (1978) estimate of 3.9 million illegals for 1973
(with a subjective 68-percent confidence range of 2.9 to 5.7 million) represents
the first carefully drafted, empirical study designed to estimate the number of
illegal aliens in the country for a year since 1970. The Lancaster-Scheuren
estimates are based on a record match of the Census Bureau's March 1973 Current
Population Survey (CPS) with IRS individual income tax records and Social Security
Administration (SSA) earnings and.benefit data. Each household matched is
categorized according to whether or not it contains: (a) an income tax filer;
(b) a worker covered by Social Security; and (c) a Social Security beneficiary.
On the basis of these three variables, every matched case can be placed in one
of seven cells in a 2x2x2 table. Log-linear models are used to estimate the
missing eighth cell representing persons not in any of the three SSA/IRS systems.
The aggregate of all eight cells (presumed to include illegal residents) is then
compared with the Census Bureau's population estimate (which presumably does not
include illegal residents) and the difference is taken as an estimate of illegal
Lancaster and Scheuren have identified possible shortcomings of their
estimation procedures in their paper and in various public forums. These include:
(a) failure to match the records properly; (b) coverage errors for both legal
and illegal residents in the Current Population Survey, including non-response,
which would affect the matching process; (c) problems in deriving the administrative
totals used as "marginals" in the 2x2x2 table; (d) problems in deriving the
independent CPS population totals; and (e) the possibility that illegal residents
are seriously under-represented in the data. These limitations are quite severe
and led the authors to characterize their work as exploratory.
There are further problems with the Lancaster and Scheuren work which the
authors do not cite. The basic data for analysis, including those used in the
application of the log-linear model, were smoothed. While it was necessary to
smooth the data to "stabilize" the estimates, it is possible that the particular
smoothing model chosen by Lancaster and Scheuren was too parsimonious to represent
all of the existing relationships correctly. More important, however, is the fact
that two of the variables chosen for analysis are highly dependent-covered Social
Security employment and tax returns filed. The relationship between these variables
is very strong and the overall estimate is very sensitive to this relationship.
Furthermore, the strength of this relationship for the whole population aged 18
to 44 is modelled by the very small group of persons in households receiving Social
Security benefits (less than 2.5 percent of this population). This group is assumed
to represent the other 97.5 percent of the population; this is an assumption which
could easily be invalid. In addition, since this group is so small, particularly
considering the number of sample cases, the true relationship could be half or
twice as great, and the relationship in the remaining population could be quite
different. Thus, the assumptions required for the estimate of 3.9 million illegal
residents to be valid are very strong indeed. Consequently, the subjective 68-percent
confidence interval cited by the authors is probably too narrow, especially for
the lower limit of the interval, and could be broadened to include the statistical
possibility of much less than 2.9 million illegal aliens in the United States.
Robinson (1979) analyzed the comparative trends in age-specific death rates
in the United States and selected groups of States from 1960 to 1975, to arrive at
crude estimates of the size of the population illegally resident in the country in
1970 and 1975. His estimates for 1975 range from 0.6 million to 4.7 million.
Robinson hypothesized that, if the illegally resident population has increased by
several million since 1960, death rates for geographic areas where illegal residents
are most concentrated should substantially exceed the rates for the general population.
This analysis depends heavily on the assumptions that: (1) few or no illegal residents
are included in decennial census counts or current population estimates; (2) the
statistics on deaths include deaths to all or nearly all illegal residents; and (3)
the death rates of illegals are similar to the death rates of other major groups in the
United States population. Robinson's analysis revealed a persistent deviation in the
trend of the death rate in 10 Southwestern and "Eastern" States since 1960, as compared
with the trend for the remaining 40 States and the District of Columbia, for young adult
(ages 20-44) white males but for no other age-sex-race group. This pattern suggests
that there was a significant number of young adult white male illegal residents in the
country in 1975. Robinson's estimates of the number of illegal residents are quite
sensitive to the underlying assumptions, as is apparent from the range offered.
Reasonable departures from the assumptions used would lead to an even wider range
Analysis of changes in the Mexican-origin population as estimated in the Current
Population Survey (CPS) suggests that the net movement to the United States from
Mexico since 1970 may be small. Heer's analysis (1979) examines the change in the
Mexican-origin population as reported in the Current Population Survey between 1970
(average of 1969 and 1971) and 1975 (average of 1975 and 1976). After adjusting
for conceptual differences between the CPS definitions of the Mexican-origin
population for these two dates, he reduces the population change by the estimated
population for these two dates, he reduces the population change by the estimated
natural increase of this population and by the amount of legal immigration. The
residual is treated as illegal immigration. Heer produces a range of estimates
by making various assumptions regarding the coverage of the Mexican-origin
population in the Current Population Survey and the rate of return migration to
Mexico. The estimated range of total illegal net immigration from Mexico for 1970
to 1975 is 0.4 million to 1.2 million; the estimates of average annual illegal net
immigration range from 82,000 to 232,000.
Heer's results should be considered only suggestive because of a number of
methodological problems in his estimation procedure. The sampling error of the
difference between the CPS Mexican-origin population at the two dates is quite
large relative to Heer's estimates of the net inflow of illegals over the period.
Indeed, the sampling error is larger than the smallest of his estimates of net
inflow. Furthermore, the annual CPS estimates of the Mexican-origin population move
in an irregular fashion, apparently because of sampling and response errors. For
example, the estimated size of this population did not change significantly between
1973 and 1977; between 1975 and 1977 there was a decrease, although this change is
not statistically significant. Had Heer carried out his analysis for the period 1973
to 1977, the results could have shown a substantial net outflow of the Mexican-origin
A range of estimates encompassing no net inflow of illegals, a net outflow, and
a large inflow could be developed on the basis of Heer's methodology by making other
appropriate and reasonable assumptions regarding the coverage of the Mexican-origin
population in the Current Population Survey. Heer's assumptions regarding levels
of coverage have no empirical basis; yet they are the primary determinant of his
estimates of illegal immigration. In fact, the uncounted Mexican-origin
population can be considered the population he is trying to estimate. Thus, if
one felt, for example, that 5 million illegal Mexican immigrants had come to the
United States between 1970 and 1975, Beer's method could be used to "demonstrate"
this by assuming that only half of the Mexican-origin population was covered in
the later Current Population Survey. Similarly, by assuming a slight or no
improvement in CPS coverage of the Mexican-origin population over time, Heer's
method could support an estimate of no net illegal movement of Mexicans into the
United States. Although Beer's study does not indicate the number of illegal
Mexican residents in the United States, it does support the conclusion that surveys
conducted by the Census Bureau do not count many illegal residents (either because
there are few of them, including the possibility that most who came earlier have
returned to Mexico, or the survey is unable to count them).
An analysis similar to Beer's can be performed on data on the number of
persons born in Mexico (a subgroup of the Mexican-origin population) from Current
Population Surveys conducted in 1969 and 1975. The change in this population
can be wholly accounted for by legal immigration and mortality. Three possible
conclusions can be drawn from these data: (1) the net flow of illegals to the
United States during 1969-75 is zero and there is no emigration of legal immigrants;
or (2) the net flow of illegals to the United States is exactly balanced by the
return movement of legal immigrants from the United States to Mexico (less than
50,000 per year); or (3) very few illegal residents are counted by survey/census
Korns (1979) conducted a study at the Bureau of Economic Analysis which bears
on the issue of the number of illegal residents in the United States although
the study was not designed to measure it. The number of persons employed in
non-agricultural activities was estimated from two different data systems. One
system, a payroll survey, bases its estimates on reports from employers and their
tax returns, data which presumably include illegal aliens. The other system, the
Current Population Survey, uses household interviews, which are assumed not to
include illegal residents. Korns then analyzes the difference between the two
systems, finding that it increased by almost two million persons between 1964
and 1970 but has not changed since 1970. He speculates that this pattern may
reflect illegal immigration but considers the data "too fragmentary" to confirm
Estimates of the Mexican Government
The Mexican government has conducted several surveys in Mexico designed to
measure the volume and characteristics of immigration to the United States. Border
surveys conducted in October-November 1977, August 1978, and May 1979 collected
migration histories from persons returned to Mexican border points by the U.S.
Immigration and Naturalization Service. Particular attention was paid to duration-
of residence in the United States and whether previous return trips to Mexico
were voluntary or the result of expulsion by U.S. authorities. The first border
survey found that much of the movement to the United States results in stays
of short duration and that a large proportion of return movement to Mexico is
voluntary (Mexico, CENIET, 1979a).
In the analysis of the first border survey, a model was designed employing
data on INS apprehensions and parameters on the voluntary/forced nature of returns
to Mexico and duration of residence in the United States. The validity of the
model is dependent on the representativeness of the Oct.-Nov. 1976 "sample" of
apprehended persons with regard to the volume and nature of movement between the
United States and Mexico during 1970-77 and the validity of the projections of
return migration to Mexico. Several estimates were prepared covering a
range of assumptions; according to the estimates the number of Mexican illegals
in the United States at the beginning of 1977 was between 0.5 and 1.2 million.
Examination of the survey results indicates that the ratios of voluntary return
migration to expulsions for the last year (1977) may be biased downward. Since these
ratios serve as the take-off point for extrapolating future migration ratios, such a
bias would result in underestimation of future return migration and consequently of
current numbers of illegal residents. A recalculation of the estimates by the
Census Bureau on the basis of alternative reasonable assumptions on projected return
migration to Mexico increased the range of the estimates to 0.7 to 2.2 million.
The Mexican government recently completed an analysis of a National Survey of
Emigration conducted in December 1978-January 1979 (Mexico, CENIET, 1979b). In one
phase of this survey, respondents in Mexico were asked if any member of the household
over 15 years of age was currently in the United States either working or looking for
work. No distinction was made with regard to the legal status of the resident in the
United States. The number of Mexicans over 15 in the United States (legally or
illegally) who were either working or looking for work in January 1979 was estimated
to be about 400,000. This type of survey estimate obviously tends to understate the
Mexican illegal alien population in the United States for a number of reasons.
Some migrants (children under 15, adult dependents) are not reported and households
all of whose members migrated to the United States are omitted completely. There
would be a reluctance on the part of Mexican householders to report illegal residents
in the United States and a tendency not to report a household member who would
soon be returning after a short absence, or a person who had established "permanent"
residence in the United States. However, the Mexican researchers contend that
the amount of bias from sources such as these is small.
A more important source of bias is the fact that the survey was conducted
in December and January. At that time of year many Mexicans who live in the
United States for much of the year would be visiting their families and relatives
in Mexico for the Christmas-New Year holiday season. Also, agricultural employment,
which draws many Mexicans to the United States, is at a seasonal low during the
months of December and January.
Even given the expectation that the estimated number of Mexican nationals
in the United States in December would be seasonally low, the estimate provided
by the Mexican National Survey seems improbably low. This conclusion is strongly
suggested by the fact that nearly one million (939,000) Mexicans registered under
the INS Alien Address Program in 1977 as permanent residents, of which perhaps a
half million were in the labor force, and that about a half million (480,000) Mexicans
were legally admitted to the United States between July 1969 and October 1977. One
interpretation to be made from a comparison of these numbers with the Mexican survey
estimate of 400,000 (which, in principle, includes both legal and illegal residents)
is that the Mexican survey succeeded in counting only those Mexican nationals who
were legal residents of the United States. Another possibility for explaining the
difference between the INS alien registration figure and the Mexican survey figure
is that many persons admitted legally to the United States, even those who register
annually with INS, actually live in Mexico much of the year. (For an example, see
Reichert and Massey, 1979.)
The results of the Mexican National Survey are also subject to sampling
error. An estimate of sampling variance for the survey figure has not been made
available, but it is apparent that even the upper 95-percent confidence limit would
yield a small number of Mexican nationals residing in the United States.
Additional information pertinent to ascertaining the number of illegal
residents in the United States will be provided in reports on a later phase of the
Mexican National Survey dealing with persons who were living in Mexico at the time
of the survey but had spent some time in the United States in the previous five
years. These "flow" data on movement between the United States and Mexico over
a 5-year period, although collected in the month of December, should permit the
direct calculation of estimates of the size and increase in unlawful Mexican
residents in the United States over the 5 years prior to the survey date. These
estimates would still omit, and have to be adjusted for, persons less than 15 years
of age, adult dependents, and the migration histories of persons in the United
States at the time of the survey.
Discussion of Analytic Estimates of the Number of Illegal Mexican Residents
The several studies described concerning illegal residents suggest that, in
spite of the evidence of a considerable gross in-movement, the numbers of illegal
residents in the United States, particularly Mexican illegals, are considerably
smaller than the numbers often cited in the news media. The picture of migration to
the United States from Mexico painted by these studies is one of a great deal of
movement to the United States offset by almost as much movement back to Mexico. The
movement across the U.S.-Mexico border (by illegal and legal migrants) appears to be
periodic or circular and tied to economic opportunities in both countries, particularly
those in the United States. This back-and-forth movement seems to be a regular part
of the functioning of the labor-intensive segment of the economy in the Southwestern
United States. It does not appear that there is a substantial amount of permanent
movement of illegals to the United States from Mexico. Focusing on apprehensions by
INS, which have often served as the basis for "extrapolation" to estimates of illegal
residents, would seem to overlook the substantial amount of movement back to Mexico
on the part of the illegal immigrants into the United States.
Reichert and Massey (1979) conducted an intensive field investigation of a
village in Michoacan, a Mexican State which has consistently been one of the
heaviest contributors to the migratory flow to the United States. Nearly three-
fourths of the 465 families in the village depended on income from migrants (legal
and illegal) to the United States, notwithstanding the considerable distance from
the United States. Almost two-thirds of the migrants were legal migrants and
roughly 90 percent of these legal migrants returned to the village within one
year of leaving. Since 1910, only 70 persons, including legals and illegals,
migrated to the United States from this village and stayed permanently in the
United States. Thus, this study supports the hypothesis that the flow of migrants
to the United States from Mexico is quite large, but that the return flow to Mexico
is very nearly as large, with the consequence that the net movement to the United
States is quite small. We should not assume, however, that the village Reichert
and Massey studied is precisely representative of the general Mexican experience.
One useful way to evaluate the credibility of assertions that many millions
of Mexicans have migrated illegally to the United States in the 1970's to take up
permanent residence is to consider the demographic realities in Mexico. A consistent
finding in studies dating back to 1930 is that well over half of the illegal
migrants to the United States from Mexico come from 5 States in central Mexico
(Guanajuato, Jalisco, Michoacan, San Luis Potosf, and Zacatecas) and the border
State of Chihuahua (Dagodag, 1975; Bustamante, 1978). The various studies also
indicate that most of the illegal migrants are young adult males (e.g., aged 15-44),
showing about 75 percent in this category (Dagodag, 1975; North and Houstoun, 1976;
Reichert and Massey, 1979). The 1970 Census of Mexico counted about 2.2 million
males in these ages in the six States; an estimate for this population in 1975 is
2.7 million. If we assume arguendo that 4.0 million Mexicans were living illegally
in the United States in 1975 and that 75 percent of them were young adult males,
and if, further, we assume that 60 percent of the Mexican illegals came from the
six Mexican States noted, as would be suggested by the available studies, we arrive
at an "estimate" of 1.8 million young adult male illegals from the six States
(4 million x 75 percent x 60 percent). This figure implies that two-thirds
(1.8 million compared to 2.7 million) of the young adult male population of these
States has moved to the United States. There is no evidence of this kind of
labor shortage in Mexico and we are led to reject an estimate of 4 million Mexican
illegals as being much too high. (For another hypothetical evaluation of the
demographic effect of illegal immigration on the population of Mexico, see
Roberts et al., 1978.)
Discussion of Non-Mexican Illegal Immigration
As inconclusive as the studies of illegal immigration from Mexico are, even
less is known about the volume of illegal immigration from areas other than Mexico.
The analytic studies provide little solid information on the number of non-Mexican
illegals and there are no numerical studies relating to specific other nationalities.
One feature that seems to distinguish the non-Mexican illegal resident population
from the Mexican illegal resident population is the mode of entry into the United
States. While most Mexican illegals enter the country without inspection ("EWI's"),
it is believed that the non-Mexican illegal is as likely to have entered the country
with legal status and then overstayed the terms of the visa ("visa abusers"), or have
entered the country with fraudulent documents ("fraudulent entrants").
It is known that there are many EWI's and persons entering with fraudulent
documents from such places as Haiti, Dominican Republic, and South America, as
well as a few from Asia, particularly the Philippines, and Africa. Considering
the distances involved, it is likely that such persons stay in the United States
longer and return home much less frequently than their Mexican counterparts. The
obstacles to clandestine entry from places other than Mexico (and Canada), together
with demographic realities, would indicate that the number of non-Mexican EWI's
and fraudulent entrants who are in the United States on a permanent basis would
be less than a few million. However, the evidence is not at all conclusive.
The largest group of non-Mexican illegal residents probably consists of
persons who have overstayed their visas or otherwise violated the terms of their
visas. This group may well number in the millions, depending on how strict a
definition of violation is used. The available studies do not provide definitive
information on the number of visa violators. However, this group differs
qualitatively from other illegal aliens in that they have had contact with the
Federal government and there is a record of the contact. Increased reliability
of reporting and recording of immigration records together with more sophisticated
computer operations and statistical procedures could lead to a continuing measure
of the size of this component of the illegal alien population.
Like the uncertainty surrounding the number of illegal residents in the
United States, little is known about the geographic distribution of the illegal
resident population. It is generally believed that most Mexican illegals are
found in the Southwestern States (e.g., Texas, California), while most non-
Mexican illegals tend to settle in the urban areas of the populous mid-Western
and Eastern States (e.g., Illinois, New York). One would expect illegal immigrants
to tend to go to such areas to live and seek work, since their "foreign" traits
would be less likely to arouse suspicion and to make them easy targets for
apprehension by immigration officials in such areas. For example, Robinson's
(1979) estimation of the size of the illegally resident population, based on the
analysis of trends in age-specific death rates in 5 Southwestern States taken
together (Texas, Colorado, New Mexico, Arizona, California), 5 "Eastern"
States (New York, New Jersey, Illinois, Michigan, Florida), and the remaining
States as a group support this general belief. These States are the intended
destinations of the large majority of legal immigrants according to INS data and
these States in general have relatively large concentrations of persons who are
of Hispanic-origin, foreign-born, or non-English mother tongue (table 2).
The geographic distribution of the legally resident alien population and the
distribution of the illegally resident population do probably resemble one another.
However, there is currently little empirical basis for determining the degree of
similarity. The one piece of evidence which suggests that settlement patterns
of legal alien residents and illegal residents are similar comes from the recent
Mexican National Survey of Emigration. As shown in table 3, the State-of-residence
distribution of Mexicans living in the United States according to the survey is
in close agreement with the distribution by State of intended residence of (legal)
Mexican immigrants to the United States based on INS data. However, since the
Mexican figures include legal as well as illegal residents, some resemblance of
the two distributions is to be expected.
This review of the existing studies on illegal residents in the United States
finds that there are currently no reliable estimates of the number of illegal
residents in the country or of the net volume of illegal immigration to the United
States in any recent past period. Even if we disregard the more conjectural of
the estimates of illegal residents (Lesko Associates, 1975; Chapman, INS, 1976a;
Guss, INS, 1977; Castillo, INS, 1978), we cannot confidently accept the results
of the analytic and empirical studies. They characteristically depend on broad
untested assumptions and are subject to other major limitations mentioned in the
paper. Often alternative reasonable assumptions could be employed which could
substantially modify the estimates and could produce an impracticably wide range.
This comment characterizes the Lancaster-Scheuren estimates, the Robinson estimates,
the Heer estimates, and the estimates based on the Mexican Border Survey. The
estimate of Mexican nationals in the United States based on the Mexican National
Survey of Emigration of December 1978-January 1979 (0.4 million) is a "direct"
estimate but it too is subject to serious question. The results would tend to
be biased downward for several reasons, including particularly the choice of
December-January as the survey period and the failure to reflect the large number
of legal immigrants from Mexico in the United States who register with INS.
Even discounted estimates of illegal residents from Mexico made by local authorities
would quickly cumulate to far larger global estimates than the Mexican National
Although the number of illegal residents in the United States remains
uncertain, the authors are willing to make some inferences from the available
studies with regard to the possible magnitude of the numbers. They offer the
following cautious speculations. The total number of illegal residents in the
United States for some recent year, such as 1978, is almost certainly below 6.0
million, and may be substantially less, possibly only 3.5 to 5.0 million. The
existing estimates of illegal residents based on empirical studies simply do not
support the claim that there are very many millions (i.e., over 6 million) of
unlawful residents in the United States.
The available evidence indicates that the size of the Mexican population
living illegally in the United States is smaller than popular estimates suggest.
The Mexican component of the illegally resident population is almost certainly less
than 3.0 million, and may be substantially less, possibly only 1.5 to 2.5 million.
The gross movement into the United States of Mexican illegals is considerable,
as is reflected in the large numbers of apprehensions made by INS, but this
"immigration" is largely offset by a considerable movement in the opposite
direction. This circular flow of legal and illegal immigrants has historically
been a normal part of the labor-intensive segment of the economy in the Southwestern
United States. Apparently, most of the Mexican nationals who enter the United
part of the year and would remain in Mexico if economic conditions deteriorated
sufficiently in the United States or if economic conditions in Mexico improved
The estimates given here suggest that the non-Mexican component of the
illegally resident population makes up a much larger share of the total number
of illegals than commonly believed. These illegals typically overstay their
visa periods or enter with fraudulent documents rather than entering
clandestinely. This group of illegals, particularly those living in the East,
is likely to differ in several significant aspects from their Mexican counter-
parts. Non-Mexican illegal residents are less likely to move back and forth
across the border in response to their short-term economic goals or fluctuations
in the U.S. economy; they are also less likely to be tied to seasonal agricultural
employment. Because of the difficulties of reentering the country or the greater
costs of returning to their homeland, it is likely that a large proportion of
the non-Mexican illegal residents remain here indefinitely. Hence, non-Mexican
illegal immigration may add to the permanent resident population to a far greater
extent than the Mexican migration flows.
We have unfortunately been unable to arrive at definitive estimates of the
number of illegal residents in the United States or of the magnitude of the illegal
migration flow. The phenomenon we have sought to measure, by its nature, is
not an easy one to deal with. Researchers and policymakers will have to live with
the fact that the number of illegal residents in the United States cannot be closely
quantified. Therefore, policy options dependent on the size of this group must
be evaluated in terms which recognize this uncertainty.
Bustamante, Jorge A., 1978. Proceedings, Brookings Institution-El Colegio de
Mexico Symposium on Structural Factors in Mexican Migration and Caribbean
Basin Migration, Washington, D.C., Brookings Institution, June 28-30, 1978.
Castillo, Leonel, 1978. Statement before the House Select Committee on Population,
House of Representatives, Ninety-Fifth Congress, Second Session, Washington, D.C.,
April 6, 1978, pp. 497-515.
Chapman, Leonard F., 1976a. Statement before the Subcommittee on Immigration and
Naturalization of the Committee on the Judiciary, United States Senate, Ninety-
Fourth Congress, Second Session, Washington, D.C., March 17, 1976.
Chapman, Leonard F., 1976b. Address before the Michigan Associated Press Editorial
Association, Gaylord, Michigan, June 11, 1976 (attachment).
Dagodag, W. Tim, 1975. "Source Regions and Composition of Illegal Mexican
Immigration to California," International Migration Review, Vol. 9, No. 4,
Winter 1975, pp. 499-511.
Goldberg, Howard, 1974. "Estimates of Emigration from Mexico and Illegal Entry
into the United States, 1960-1970, by the Residual Method," unpublished graduate
research paper, Center for Population Research, Georgetown University,
Washington, D.C., 1974.
Guss, Edward Jon, 1977. "Even If You're On the Right Track, You'll Get Run Over
If You Just Sit There," I and N Reporter, Vol. 25, No. 4, Spring 1977, pp. 52.
Heer, David M., 1979. "What is the Annual Net Flow of Undocumented Mexican
Immigrants to the United States?," Demography, Vol. 16, No. 13, August 1979,
Keely, Charles B., 1977. "Counting the Uncountable: Estimates of Undocumented
Aliens in the United States," Population and Development Review, Vol. 3,
No. 4, December 1977, pp. 473-481.
Korns, Alexander, 1979. "Cyclical Fluctuations in the Difference Between the
Payroll and Household Measures of Employment," Survey of Current Business,
Vol. 59, No. 5, May 1979, pp. 14-44, 55.
Lancaster, Clarice and Frederick J. Scheuren, 1978. "Counting the Uncountable
Illegals: Some Initial Statistical Speculations Employing Capture-Recapture
Techniques," 1977 Proceedings of the Social Statistics Section, Part I,
pp. 530-535, American Statistical Association, 1978.
Lesko Associates, 1975. Final Report: Basic Data and Guidance Required to
Implement a Major Illegal Alien Study During Fiscal Year 1976, prepared for
Office of Planning and Evaluation, U.S. Immigration and Naturalization Service,
Washington, D.C., October 1975.
Library of Congress, Congressional Research Service, 1976. "Critique of
the Estimates of the Number of Illegal Aliens in the United States Made by
Lesko Associates," Memorandum from Dennis L. Little, Daniel Melnick, and
Joyce Vialet to Honorable Herman Badillo, January 23, 1976.
Mexico, Centro Nacional de Informaci6n y Estadfsticas del Trabajo, 1979a.
El Volumen de la Migraci6n de Mexicanos no Documentados a los Estados Unidos:
Nuevas Hip6tesis, by Manual Garcia y Griego, December 1979.
Mexico, Centro Nacional de Informaci6n y Estadfsticas del Trabajo, 1979b.
Los Trabajadores Mexicanos en los Estados Unidos: Primeros Resultados
de la Encuesta Nacional de Emigraci6n, by Carlos H. Zazueta and
Rodolfo Corona, December 1979.
Morris, Milton, and Albert Mayio, 1980 forthcoming. Foreign Policy Aspects of
Illegal Immigration, esp. Chapter III, "Levels and Sources of Illegal
Immigration," Washington, D.C., Brookings Institution, 1980 forthcoming.
North, David S., and Marion F. Houstoun, 1976. The Characteristics and Role
of Illegal Aliens in the U.S. Labor Market: An Exploratory Study, report
prepared for the U.S. Department of Labor, Linton and Co., Washington, D.C., 1976.
Reichert, Josh, and Douglas S. Massey, 1979. "Patterns of U.S. Migration from a
Mexican Sending Community: A Comparison of Legal and Illegal Migrants,"
International Migration Review, Vol. 13, No. 4, Winter 1979, pp. 599-623.
Roberts, Kenneth, Michael E. Conroy, Allan G. King, and Jorge Rizo-Patron, 1978.
The Mexican Numbers Game: An Analysis of the Lesko Estimate of Undocumented
Migration from Mexico to the United States, Research Report, Bureau of
Business Research, The University of Texas at Austin, April 1978.
Robinson, J. Gregory, 1979. "Estimating the Approximate Size of the Illegal
Alien Population in the United States by the Comparative Trend Analysis of
Age-Specific Death Rates," unpublished paper presented at the annual meeting
of the Population Association of America, Philadelphia, Pennsylvania,
April 26-28, 1979.
U.S. Department of Justice, Immigration and Naturalization Service, 1976a.
"Fraudulent Entrants Study: A Study of Malefide Applicants for Admissions
at Selected Airports and Southwest Land Border Parts" (Illegal Alien
Study--Part 1), September 1976.
U.S. Department of Justice, 1976b. Domestic Council Committee on Illegal Aliens:
Preliminary Report, Washington, D.C., 1976.
Warren, Robert, 1979. "Estimates of the Mexican-Born Undocumented Population
Under 25 Residing in the U.S. in 1970 (with a Methodology Applicable to
1980)," unpublished graduate research paper, Department of Sociology,
Catholic University, Washington, D.C., 1979.
Table 1. Estimates of Aliens Illegally Residing in the United States for
Various Dates in the 1970's
(Numbers in parenthesis represent publication date)
Mexican-origin population only
Ages 18 to 44 only. The numbers
in parenthesis represent rough
68 percent confidence limits
The Lesko report also contains an
estimate of 5.2 million for the
White male population, ages 20
to 44 only
Net illegal immigration of
Mexican-origin population only
Mexican-origin population only
Mexican nationals over 15 years
of age working or looking for
work, without regard to legal
Chapman, INS (1976a)
Guss, INS (1977)
(1979a) by U.S.
Bureau of the
Castillo, INS (1978)
Table 2. Characteristics of the Population of Selected State Areas:
5 "Eastern" States, 5 Southwestern States, and the 41 Other States
Distribtion of Desnated Residence of Alen
Population by Area, 1970 Tmr a 1 97-77 evidence of
population on-English immigrants Alien
Area 1970 Census Spanish Foreig Mother All Born in Populatign,
(000's) Heritage-'/ Born- Tonguer Immigrants Mexico4/ 19772-
5-Southwestern States /
a/ Includes District of Colum(ia.
b/ New York, New Jersey, Illinois, Michigan, Florida
c/ Texas, Colorado, New Mexico, Arizona, California
I/ U.S. Bureau of the Census
2/ U.S. Bureau of the Census
3/ U.S. Bureau of the Census
4/ Immigration and Naturaliz
5/ Immigration and Naturaliz.
1970 Cens of PopulatLon,
1970 Censs of Population,
e, Annual Re
e, Annual Re
Table 3. Geographic Distribution of Mexican Alien Residents in the United States
According to Data From the Mexican National Survey of Emigration and
the Immigration and Naturalization Service
Mexican National Survey Data: INS Data: State of Intended
State of Residence of Mexican Residence of Mexican Aliens
Nationals in the United States, Admitted to the United
Dec. 1978 Jan. 19791/ States, 1976-772/
State Number Percent Number Percent
NOTE: The Mexican Survey figures include legal as
over 15 years of age working or looking for
legal immigrants only.
well as illegal Mexican residents
work; the INS data pertain to
Sources: 1/ Mexico, Centro Nacional de Informaci6n y Estadisticas del Trabajo.
Los Trabajadores Mexicanos en los Estados Unidos: Primeros Resultados
de la Encuesta Nacional de Emigracidn, by Carlos H. Zazueta and
Rodolfo Corona, December 1979.
2/ Immigration and Naturalization Service, Annual Report, 1977, table 12A.
1 UNITED STATES DEPARTMENT OF COMMERCE
Bureau of the Census
Washington. D.C. 20233
January 31, 1980
Attached is a copy of a paper prepared by staff members of the
Bureau of the Census as a working document at the request of the
research staff of the Select Commission on Immigration and Refugee
Policy. It reviews existing studies of the number of illegal
residents in the United States and, because of wide interest in
the subject, is being made available to the general public.
The authors, Jacob S. Siegel, Jeffrey S. Passel, and J. Gregory
Robinson, stress in their paper that there are no reliable estimates
of the numbers of illegal residents in the country or of the net
volume of illegal immigration to the United States in any recent
past period. They further note that even the several analytic
studies now available which they review are subject to major
limitations. The estimates of the authors given in the "Conclusion"
are not based on empirical research but represent their own specu-
lations based on studies conducted generally by others. They are
not official estimates of the Census Bureau.
The paper is entitled "Preliminary Review of Existing Studies of the
Number of Illegal Residents in the United States."