When the definitive history of Joe Biden’s presidency is written, it will likely conclude that his most important legacy was unprecedented immigration levels. The U.S. foreign-born population (legal and illegal) grew by 8.3 million during his time in office, driven mostly by the border crisis that he instigated.

At first, the media called the massive influx “seasonal” or blamed it on pent-up demand after COVID lockdowns. Later, they claimed that every Western country faced a similar situation. Many argued it was inevitable. As David Bier of the Cato Institute argued in the New York Times, “Biden Can’t Stop Immigration. Time to Embrace It.” Of course, nothing about it was inevitable.

The border crisis began almost immediately after Biden took office. The president’s public statements and his decisions to end the Remain in Mexico policy, dismantle Title 42, and terminate agreements with Central American countries that stopped people from transiting through those nations all played a part. Biden also chose not to pressure Mexico to stop the flow of migrants through its territory. Most important, however, was the administration’s decision to release most people encountered at the border into the United States rather than detain them and expedite their removal.

The resulting surge at the U.S. border was inevitable. Illegal immigrants are like anyone else—they respond to incentives. Under Biden, they realized that if they made it to the border, the chances of being released into the U.S. were very high. Millions responded to that incentive and came. Nationwide “border encounters” surged from about 70,000 a month at the end of 2020 to about 200,000 a month by the end of 2021, increasing thereafter to an average of over 300,000 by the second half of 2023.

In the summer of 2024, Biden and his advisors belatedly realized the political damage the border was causing. The administration adopted some modest reforms, including tightening asylum rules, increasing detention and removal, and pressuring Mexico. Encounters dropped by more than half, though the administration also restarted the CHNV program, which flew people from Cuba, Haiti, Nicaragua, and Venezuela into the United States without visas so they could apply for asylum.

All of this stopped when Donald Trump returned to office and announced a series of policy changes to reduce illegal immigration. First, his administration began detaining almost everyone encountered at the border, making detention and expedited removal the norm. The CHNV program was terminated, as was the CBP One app, which the Biden Administration set up to allow illegal immigrants to initiate the asylum process even before they arrived on U.S. soil. The Trump Administration also put pressure on Mexico to interdict people before they reached the border. By April 2025, encounters were just 3% of what they had been in April 2024.

What the Numbers Say

While border statistics convey the urgency of the problem, they cannot tell us the number of illegal immigrants who live in the U.S.

Whenever an illegal immigrant gets caught trying to cross or presents themselves at a port of entry, it is counted as an encounter. But encounters often involve the same person caught more than once. Furthermore, even under Biden, some share of those stopped at the border were detained and sent home. To estimate the size and growth of the illegal population, we need to examine other data.

Several Census Bureau surveys ask respondents about their place of birth and citizenship, among other things, allowing us to identify the number of “foreign-born” living in the United States, including naturalized citizens, permanent residents, guest workers, foreign students, and illegal immigrants. The large American Community Survey (ACS) is ideal for estimating the size of the foreign-born population. The 2025 data will be released soon, but they will reflect the population only through July 2025, telling us little about the impact of Trump’s policies.

Alternatively, the smaller Current Population Survey (CPS) is conducted monthly and is released almost immediately. The CPS shows an unprecedented decline of 2.9 million in the total foreign-born population from January 2025 to July 2026.

As for the illegal alien population, my colleague Karen Zeigler and I estimate from CPS data that it has fallen by 2.3 million since January 2025—from 15.8 million at the start of the Trump Administration to 13.5 million through July 2026. By contrast, it had grown by an unprecedented 5.6 million during Biden’s time in office. These estimates include adjustments for those the CPS missed.

Like most researchers in this field, arriving at these numbers requires taking the total foreign-born population in the CPS, subtracting the number of likely legal immigrants in the U.S., which we know from administrative data, and then adjusting for undercount. The estimated increase or decrease represents a net change, not the number of new arrivals. New additions are offset by those who “exited” the illegal immigrant population, including deaths, outmigration (including deportations), or legalizations through grants of asylum, marrying an American, or winning the visa lottery.

As a result, the total number of new illegal immigrants who arrived during Biden’s four years was likely eight million or more. (The illegal population did not grow by this amount because some left the country. And of course, all children born to illegal immigrants are part of the native-born population, not the illegal population.)

In the current environment of stepped-up enforcement, some of the recent decline in the overall foreign-born population, particularly among illegal immigrants, might reflect an increased unwillingness to take part in the CPS rather than people departing the country. But we found no clear evidence of this: CPS response rates have declined in recent years, but the trend long predates Trump and Biden and is not correlated with the size of the foreign-born population. Moreover, the share of respondents declining to answer immigration-related questions remains low and unchanged.

For those still skeptical that the CPS could include millions of illegal immigrants, consider that the growth of the total foreign-born population under Biden can be explained only by illegal immigration. Legal immigration was nowhere near that high during his administration. Since the 8.3 million increase is a net figure, it is likely that 12 million new people—legal and illegal immigrants—and perhaps more settled in the country during his four years in office.

While the growth in the overall foreign-born population and the illegal population was historic between 2021 and 2024, it is extremely unlikely that there are 20 million, 30 million, or more illegal immigrants in the U.S., as some contend. If the illegal population were well beyond what the Census Bureau surveys indicate, it would show up in several ways, including in birth records. Reconciling a much larger illegal immigrant population with the number of births recorded each year would require accepting two unsound assumptions: either that illegals have unusually low fertility rates or that they are not registering the births of their children and securing U.S. citizenship for them.

A significant share of those who crossed the border were school-age, which has certainly affected enrollment in some places. But nothing in the overall school enrollment data indicates that the number of illegal immigrants is millions higher than what the survey data indicate. The same is true of the data we collect from surveying employers.

This does not mean that illegal immigration has not added 20 million or more people to the country. The estimated illegal immigrant population, combined with their five to six million U.S.-born minor children and several million U.S.-born adult offspring, totals around 20 million, even though the number of illegal immigrants currently living in the country is below 20 million.

Furthermore, the 1986 amnesty legalized nearly three million people, and over the following decades millions of other former illegal immigrants have “adjusted their status”—that is, they were given green cards through asylum grants, marriages to Americans, visa lottery wins, and so on. The total number of current and former illegal immigrants, plus their U.S.-born offspring, could in fact easily be 30 million. Although the number of current illegal immigrants is not as big as some argue, the sense that illegal immigration has a much bigger impact on society is not misplaced.

The drop in border encounters and decline in immigrants in the CPS have invalidated two of the most common assertions made by illegal immigration apologists: that illegal immigrants would keep coming no matter the policies in place and that those already here cannot be induced to leave. The enormous decline in border encounters demonstrates how wrong the first assertion was. And the dramatic decline in the number of illegals in the country under the Trump Administration is powerful evidence that the second was wrong as well.

Though the CPS is not perfect, nor is our estimate of illegal immigrants based on it, it is the best and most up-to-date data we have. It showed a huge increase in the foreign-born population during the Biden-induced border surge and now shows a big falloff under the Trump Administration. This is consistent with the huge decline in border encounters, a sharp increase in deportations, data showing fewer legal immigrants being admitted, and numerous media reports of illegal immigrants going home.

As best we can tell, the decline in the foreign-born population is real. It is due primarily to policy changes that have significantly reduced the number of people coming to the U.S. and increased the number of people leaving.

None of this should be terribly surprising. In the same way illegal immigrants responded to the incentives created by the Biden Administration by coming in enormous numbers, they are now responding to incentives created by the Trump Administration: fewer are coming, and more are leaving. It turns out immigration can be controlled when the political will exists to do so.