Numbers, Facts and Trends Shaping Your World

Estimates of the Size and Characteristics of the Undocumented Population


The Pew Hispanic Center has developed new estimates for the size and key characteristics of the population of foreign-born persons living in the United States without proper authorization using data from the March 2004 Current Population Survey which is conducted by the U.S. Census Bureau and the Department of Labor.

Major findings include:

  • Following several years of steady growth, the number of undocumented residents reached an estimated 10.3 million in March 2004 with undocumented Mexicans numbering 5.9 million or 57 percent of the total.
  • As of March 2005, the undocumented population has reached nearly 11 million including more than 6 million Mexicans, assuming the same rate of growth as in recent years.
  • About 80 to 85 percent of the migration from Mexico in recent years has been undocumented.
  • Since the mid-1990s, the most rapid growth in the number of undocumented migrants has been in states that previously had relatively small foreign-born populations. As a result, Arizona and North Carolina are now among the states with the largest numbers of undocumented migrants.
  • Although most undocumented migrants are young adults, there is also a sizeable childhood population. About one-sixth of the population–some 1.7 million people– is under 18 years of the age.

Size and Origins of the Undocumented Population

Neither the Census Bureau nor any other U.S. government agency counts the unauthorized migrant population or defines their demographic characteristics based on specific enumeration. There is, however, a widely-accepted methodology for estimating the size and certain characteristics, such as age and national origins, of the undocumented population based on official data. This methodology essentially subtracts the estimated legal-immigrant population from the total foreign-born population and treats the residual as a source of data on the unauthorized migrant population (Passel, Van Hook, and Bean 2004).

The estimates reported here use this methodology with data from the March 2004 Current Population Survey (CPS). The CPS, a monthly survey of about 50,000 households conducted jointly by the U.S. Bureau of Labor Statistics and the Census Bureau, is best known as the source for monthly unemployment statistics. Every March both the sample size and the questionnaire of the CPS are augmented to produce the Annual Social and Economic Supplement which provides additional data on several additional subjects including the foreign-born population.

As of March 2004, there were an estimated 10.3 million unauthorized migrants living in the United States. A comparison to past estimates derived with the same methodology shows that the undocumented population has grown rapidly in recent years. There were 8.4 million unauthorized migrants living in the United States in April 2000 according to estimates derived from Census 2000 (Passel, Van Hook, and Bean 2004). Thus, average annual growth over the 4-year period since 2000 was about 485,000 per year. Assuming this rate of growth held steady, the best estimate for March 2005 points to a figure of somewhat less than 11 million for the number of undocumented residents.

The 10.3 million undocumented migrants in March 2004 represent about 29 percent of the almost 36 million foreign-born residents of the United States (Figure 1).

Mexicans make up by far the largest group of undocumented migrants at 5.9 million or 57 percent of the total in the March 2004 estimates. This share has remained virtually unchanged for the past decade, even as the size of the undocumented population has grown very rapidly. In addition, another 2.5 million undocumented migrants or about 24 percent of the total are from other Latin American countries. About 9 percent are from Asia, 6 percent from Europe and Canada, and 4 percent from the rest of the world (Figure 2).

While the annual net growth of the unauthorized population has averaged roughly half a million per year since 1990, the number of new undocumented migrants reaching the country every year is significantly larger. While it grows through the arrival of new migrants, the undocumented population is reduced each year because many undocumented migrants depart, a few die, and significant numbers acquire legal status. Over the past decade the number of newly arrived unauthorized migrants added to the U.S. population has averaged 700,000–800,000 a year (Figure 3). Over the same interval, legal migrants arrived at roughly the same rate.

Overall, the Mexican-born population living in the United States, including both those with legal status and otherwise, has continued to increase dramatically. About 11.2 million Mexicans were in the United States as of March 2004 with just under half (47 percent) or about 5.2 million having legal immigration status. Mexicans overall represent about 32 percent of the foreign-born population, a high figure by historical standards but not unprecedented; both Irish and German immigrants accounted for a higher percentage of the foreign-born at various points in the mid- and late-19th century (Gibson and Lennon 1999).

The number of Mexican migrants in the United States has grown quite rapidly over the past 35 years, increasing almost 15-fold from about 760,000 in the 1970 Census to more than 11 million in 2004—an average annual growth rate of more than 8 percent, maintained over more than 3 decades. This remarkable growth has been largely driven by undocumented migration. On average the Mexican population living in the United States has grown by about half a million people a year over the past decade. Unauthorized migrants have accounted for about 80–85 percent of the increase (Figure 4). Since this growth has been fairly consistent, we estimate that the number of undocumented Mexicans in the United States reached 6 million before the end of 2004 and could surpass 6.5 million by the end of 2005.


Almost two-thirds (68 percent) of the undocumented population lives in just eight states: California (24 percent), Texas (14 percent), Florida (9 percent), New York (7 percent), Arizona (5 percent), Illinois (4 percent), New Jersey (4 percent), and North Carolina (3 percent). (See Figure 5 and Table 1.)

The appearance of Arizona and North Carolina on this list of the states with large shares of the undocumented populations highlights another recent trend. In the past, the foreign-born population, both legal and undocumented, was highly concentrated. But, since the mid-1990s, the most rapid growth in the immigrant population in general and the undocumented population in particular has taken place in new settlement areas where previously the foreign-born had been a relatively small presence.

According to estimates for 1990, about 88 percent of 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. But, by 2004, only 61 percent of the undocumented population lived in those six states.

Another way of looking at this movement is that in 1990 only about 400,000 undocumented migrants lived in the remaining 38 states and the District of Columbia (Figure 6). By 2004, an estimated 3.9 million undocumented migrants lived outside of those traditional settlement states, nearly a tenfold increase. (Table 1 shows the available estimates for all states.)

The rapid growth of the undocumented population has been the principal driver of growth in the foreign-born populations in new settlement states such as Arizona, North Carolina, Georgia, and Tennessee (Passel, Capps, and Fix 2002; Passel and Zimmermann 2001). In 17 new settlement states stretching from the northwest through the mountain states to the southeast, the undocumented make up 40 percent or more of the total foreign-born population (Figure 7). Of the six traditional settlement states, only Texas has such a large ratio of its undocumented population to the total foreign-born. In the other five traditional settlement states, undocumented migrants make up less than 30 percent of the foreign born population and in New York the undocumented share is less than 20 percent.

Demographic Composition

Although the stereotype of undocumented migrants being mostly young adult males is partly supported by our estimates, a somewhat different picture of the entire group emerges from a more detailed demographic analysis. About one in every six undocumented migrants is a child, accounting for about 1.7 million of the more than 10 million undocumented migrants (Figure 8). A relatively small percentage of the unauthorized migrants are of middle age or older with only about 1.1 million being over 40 years old and virtually none being over age 65. Among the younger adults, there is a predominance of males, but there is also a significant number of women in the undocumented population—about 3 million or 29 percent of the total. For undocumented migrants aged 18–39, nearly 60 percent or about 4.5 million are men; in this age group of the undocumented there are about 146 men for every 100 women.

An additional demographic category is important to any discussion of the undocumented population but is particularly difficult to measure—the U.S.-born children of undocumented parents. Previous work with estimates similar to those presented here has shown that there are about two such U.S.-born children of undocumented migrants for every undocumented child (Passel, Capps, and Fix 2004). Applying that ratio to the March 2004 estimates points to well over 3 million U.S.-born children in families headed by undocumented migrants.

The estimates completed to date do not give a direct measure of the number of undocumented migrants in the U.S. labor force. However, labor force participation estimates from previous research conducted with data for 2002 (Passel, Capps, and Fix 2004) would lead to an estimate of about 7 million undocumented workers for 2004—representing about 5 percent of U.S. workers.

Note on Methods and Terminology

We estimate the number of undocumented migrants by subtracting legal foreign-born residents from the total foreign-born population. This total is based on the March 2004 Current Population Survey (CPS) with an allowance for immigrants not included in the CPS. To estimate the number of legal residents, we use official data, mostly provided by the Department of Homeland Security and other government agencies, for the following categories: (a) legal permanent residents, i.e., green-card holders including amnesty recipients under the Immigration Reform and Control Act of 1986; (b) refugees, asylees and parolees; and (c) legal temporary residents, which include students, professors, high-tech workers, and a number of other temporary visa categories.

The estimates of the total foreign-born population are based on the March 2004 Current Population Survey, but have been corrected for misreporting of place of birth. In developing the estimates of undocumented migrants, we employ assumptions about the coverage of legal residents and undocumented residents in the Current Population Survey. The correction factors are based on official estimates of census undercount from Census 2000 and other research on coverage of the foreign-born.

The state-level estimates were done with two different methods. For the 6 “historical” states (CA, NY, TX, FL, IL, and NJ) and the balance of the country, the residual method described in the above was applied “directly” with a comparison of the estimated number of legal residents to the foreign-born population counted in the state. For the remaining states, we used a so-called “synthetic method.” The ratio of undocumented residents to the foreign-born population for the entire group of the remaining 44 states and the District of Columbia was applied to each state separately. Ratios were computed for four areas of origin (Mexico, Other Latin America, Asia, and the rest of the world) and for four periods of arrival (2000–04, 1995–99, 1990–94, and pre-1990).

Undocumented or unauthorized migrants are those who do not fall into any of our legal categories. Two groups account for most undocumented migrants: (a) those who entered the country without valid documents, including people crossing the Southwestern border clandestinely; and (b) those who entered with valid visas but overstayed their visas’ expiration or otherwise violated the terms of their admission. Some “undocumented” migrants in our estimate have legal authorization to live and work in the United States. Two such groups—those with temporary protected status (TPS) and asylum applicants—may account for as much as 10 percent of our estimate.

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