Illegal immigrant population of the United States

The actual size and the origin of the illegal immigrant population in the United States is uncertain and is difficult to ascertain because of difficulty in accurately counting individuals in this population. Figures from national surveys, administrative data and other sources of information vary widely.

The number of illegal immigrants peaked at about 12 million in 2007 and since that time has declined. According to the nonpartisan Pew Research Center, the estimated population of illegal immigrants in the U.S. rose rapidly in the 1990s, "from an estimated 3.5 million in 1990 to a peak of 12.2 million in 2007," then dropped sharply during the Great Recession before stabilizing in 2009. Pew estimated the total population to be 11.1 million in 2014, or approximately 3 percent of the U.S. population. This "is in the same ballpark" as figures from the United States Department of Homeland Security (DHS), which estimated that 11.4 million illegal immigrants lived in the United States in January 2012. The estimate and trends are also consistent with figures reported by the Center for Migration Studies, which reported that the U.S. illegal immigrant population fell to 10.9 million by January 2016, the lowest number since 2003.

The "residual method" is widely used to estimate the illegal immigrant population of the US. With this method, the known number of legally documented immigrants to the United States is subtracted from the reported U.S. Census number of self-proclaimed foreign-born people (based on immigration records and adjusted by projections of deaths and out-migration) to obtain the total, illegal immigrant (residual) population. This methodology is used by the US Department of Homeland Security, the Pew Hispanic Center, the Center for Immigration Studies, and the U.S. Census Bureau. Since illegal immigrants have many reasons for not answering the U.S. Census correctly and since there are no penalties for answering the U.S. Census incorrectly, it is accepted that it under-counts the number of illegal immigrants. The users of this methodology assume that 10% of illegal immigrants are not counted by census takers. The 10% assumption is based on a 2001 University of California survey asked of 829 people born in Mexico and living in Los Angeles whether they responded to census interviewers in the 2000 census with 40% of queried households refusing to answer the survey. Critics claim that the estimate is unreliable for a number of reasons: figures for outmigration are not tracked by the federal government; the proportion of illegal immigrants who respond to the Census is unknown; the estimate that 10% of illegal immigrants do not respond to the census is arbitrary and unsupported by a sufficient sample size and geographic spread; and that the self reporting of where one was born relies on the honesty of the person being questioned.

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