Research shows twice as many illegal aliens in U.S. than previously thought

There are roughly twice as many illegal aliens in the U.S. than previously thought, according to a new study by MIT Sloan School of Management’s Mohammad Fazel-Zarandi, Edward Kaplan and Jonathan Feinstein.

The study, published Friday, estimates that there are about 22.1 million illegal immigrants in the U.S.; the most prominent current estimate is 11.3 million. Even using parameters intentionally aimed at producing a conservative estimate, the study finds a population of 16.7 million illegal aliens.

“Immigration policy is a hot-button issue in the U.S. and the question of how to address undocumented immigrants—that is, the people living here without permission from the American government—provokes passion on both sides,” says Fazel-Zarandi. “The fact is, though, if you want to solve a problem, you need to know the scope of it. Debates about the amount of resources to devote to this population and the relative benefits and disadvantages of alternative policies—including deportation, amnesty, and border control—depend greatly on having a correct estimate. The number sets the scale.”

The commonly quoted estimate of 11.3 million is extrapolated from population surveys and legal immigration records. Fazel-Zarandi and his colleagues take a different tack.

“Using the best available data, including some that have only recently become available, our approach is grounded in demographic and mathematical modeling,” he says. “The results of our analysis are clear: The number of unauthorized migrants in this country is substantially larger than widely accepted previous estimates.”

The discrepancy reflects the inherent challenge of relying on survey-based methodologies to identify this hidden population. First, the approach entails reaching a representative sample of all those born outside of the U.S. Second, it requires achieving accurate responses from survey respondents to questions about where they were born, and whether they are American citizens.

“It’s very likely that undocumented immigrants are more difficult to locate and survey than other foreign-born residents, and if contacted, they might be inclined to misreport their country of origin, citizenship, and number of household residents fearing the legal consequences of revealing their status,” says Fazel-Zarandi. “Any of these circumstances would lead to underestimating the true number of unauthorized immigrants.”

For their study, Fazel-Zarandi and his colleagues use a new method that is based on operational data, such as border apprehensions, deportations, and visa overstays, and demographic data, including mortality rates and emigration rates. They combine these data using a mathematical model that estimates and tracks population inflows and outflows.

The study, which spans from 1990 to 2016, also includes estimates on unlawful border crossings based on newly available data. “We don’t know the number of people who cross the border successfully—we only know when people get caught trying because the Department of Homeland Security fingerprints every person who gets apprehended,” he says. “From the apprehension data, it’s possible to infer how many people must have tried to cross the border.”

The study finds that the number of illegal immigrants in the U.S. grew steadily throughout the 1990s and reached a peak in 2007/2008. The number of illegal aliens has since leveled off and become stable.

“We hope that our findings, while based on a number of assumptions and uncertainties, will help frame debates about policies regarding the number of undocumented immigrants in the U.S.,” says Fazel-Zarandi.