Center for Migration Studies Undercounts Illegal Alien Population



Estimating the size and distribution of the illegal alien population in the United States is an inexact science. Estimates vary from approximately 11 million to more than 20 million, depending on who you ask.

However, the Center for Migration Studies (CMS) – a New York-based organization that champions mass-immigration policies – recently released an especially low estimate of this population which should make all decent research professionals collectively scratch their heads. The organization claimed in a recent report that the illegal alien population in the United States has decreased to 10.35 million.

This report is plagued by numerous methodological errors that result in a far lower count of the illegal alien population than what it is in actuality. These include, but are not limited to:

  • The authors fail to account for significant undercount issues in their primary data source, the Census Bureau’s American Community Survey (ACS). Historically, credible organizations have acknowledged that many illegal aliens avoid responding to census surveys. Even the Pew Research Center – which also typically releases lower illegal alien population estimates – acknowledges this fact. They noted in a recent methodology that “unauthorized immigrants generally have significantly higher undercount rates [in the ACS]than lawful immigrants who, in turn, tend to have higher undercounts than the U.S.-born population.”
  • In a similar vein, the organization improperly uses ACS data to estimate how many foreign-born individuals currently reside in the United States. It’s important to estimate the total foreign-born population in the United States before the illegal alien population can be estimated. (More on why this step is important can be found here)

There are several different ACS figures that offer an indication of how many foreign-born individuals reside in the United States based on different demographic criteria. However, depending on which category is used, the results could vary by more than two million. The 2019 ACS question on citizenship yields a weighted foreign-born total that is more than two million individuals lower than the weighted results for questions regarding how long foreign-born respondents have lived in the country. For estimating the total foreign-born population, the latter question should be used so that all individuals who indicate themselves as foreign-born are included, or else the illegal alien population will be too low once all necessary calculations are made.

  • When calculating how many illegal aliens have left the country, the authors assume that essentially all migrants who are ordered removed actually leave. They also assume that few or none of those who leave or are deported return to the States in short order. While ACS data will account for most of these illegal aliens over the long term (minus those who do not respond to census surveys), it does not account for those who may have recently defied removal orders, or who may have been removed, but then subsequently re-entered the country soon thereafter.

All these issues combined explain why the CMS study’s illegal alien population was significantly lower than the estimates from essentially all other organizations. Once these methodological errors are accounted for, the figures mirror the Federation for American Immigration Reform’s most recent estimate (based largely on the same ACS data) of 14.5 million. Our methodology is explained in this study.

The CMS authors are very open and honest about what impact they hope their report will have. “The fact that the US undocumented population is shrinking,” they assert, “provides important context to media and political narratives, which often emphasize short-term ‘crises’ or claim that the United States is ‘overwhelmed’ by immigrants.”

Essentially, the authors hope that their flawed estimate will relieve the concerns of most Americans regarding the ongoing border crisis by suggesting that the United States is easily able to absorb this entire illegal alien population. And, since we are supposedly capable of absorbing this population, why not welcome them with amnesty as well?

The organization argues that “expanding legal pathways and promoting citizenship [for illegal aliens]” could “address disparities” between illegal aliens, legal permanent residents (LPRs), and citizens. In other words, it’s the age-old argument that it’s the fault of American citizens that immigration lawbreakers are unable to fully participate in the workforce, not a result of their own illegal actions.

The authors of this study believe that Americans should not just ignore the fact that illegal aliens violated our immigration laws, but they further propose that we should celebrate their actions by offering them citizenship. This demand that we reward immigration lawbreakers is nothing new. Neither is the practice of conveniently inflating or deflating statistics until they conveniently fit the pre-constructed narrative of the “researchers” involved. American citizens deserve better on both accounts.

About Author

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Spencer joined the Federation for American Immigration Reform (FAIR) in 2015. He conducts research, and writes content for FAIR’s publications and website. He brings previous experience in state politics, gubernatorial and district campaigns, and D.C. political non-profits. Spencer holds a B.A. in Government from the University of Texas at Austin.