This post has been updated.

The increase in border crossings at the U.S. border with Mexico has generated a lot of attention — and a lot of theories about where this increase is coming from and whether it might be linked to Biden administration policies.

Underappreciated in the developing narrative is just how predictable the rise in border crossings is. We analyzed monthly U.S. Customs and Border Protection data from 2012 through February and found no clear evidence that the overall increase in border crossings in 2021 can be attributed to Biden administration policies. Rather, the current increase fits a pattern of seasonal changes in undocumented immigration combined with a backlog of demand because of 2020s coronavirus border closure.

The evidence for a seasonal increase

CBP reports monthly data on how many migrants its agents apprehend at the southern border, including unaccompanied minors. The figure below shows the most recent data CBP has made publicly available.

As the blue line shows, CBP has recorded a 28 percent increase in migrants apprehended from January to February 2021, from 78,442 to 100,441.

At the same time, CBP’s numbers reveal that undocumented immigration is seasonal, shifting upward this time of year. During fiscal year 2019, under the Trump administration, total apprehensions increased 31 percent during the same period, a bigger jump than we’re seeing now. We’re comparing fiscal year 2021 to 2019 because the pandemic changed the pattern in 2020. In 2018, the increase is about 25 percent from February to March — somewhat smaller but still pronounced.

But was 2019 an aberration? In the figure below, we combine data from fiscal year 2012 to fiscal year 2020 to show the cumulative total number of apprehensions for each month over these eight years. As you can see, migrants start coming when winter ends and the weather gets a bit warmer. We see a regular increase not just from January to February, but from February to March, March to April, and April to May — and then a sharp drop-off, as migrants stop coming in the hotter summer months when the desert is deadly. That means we should expect decreases from May to June and June to July.

What we’re seeing right now is a predictable seasonal shift. When the numbers drop again in June and July, policymakers may be tempted to claim that their deterrence policies succeeded. But that will just be the usual seasonal drop.

What about unaccompanied minors?

What is more unusual at this moment is the increase in border crossings by unaccompanied minors, which appears to be more than just a seasonal pattern. This poses a more distinctive challenge for the Biden administration, although it is also possible that there will be a similar drop in crossings by minors during the summer months.

Have Biden administration policies caused this increase? There is no evidence to suggest that this is the case. Organizations working with asylum seekers and unaccompanied children in the San Diego-Tijuana border region tell us that even before President Biden took office smugglers may have been exploiting potential clients by claiming it would be easier to enter the United States once the Trump administration was gone. This is supported by the literature that examines changes in the fees smugglers charge at the U.S.-Mexico border, as well as a DHS report on the same subject

So why are we seeing more migrants so far in 2021?

CBP has indeed reported apprehending more migrants in February 2021 than in the same month in previous years. In the first figure, above, the blue trend line for fiscal year 2021 is above the orange trend line for fiscal year 2019. But 2020 was the pandemic, when movement dropped dramatically. Countries around the world closed their borders. Here in the United States, the Trump administration invoked Title 42, a provision from the 1944 Public Health Act, to summarily expel migrants trying to enter the United States without proper documentation.

In other words, in fiscal year 2021, it appears that migrants are continuing to enter the United States in the same numbers as in fiscal year 2019 — plus the pent-up demand from people who would have come in fiscal year 2020, but for the pandemic. That’s shown in the first figure, earlier, in which the blue trend line for the five months of data available for fiscal year 2021 (October, November, December, January and February) neatly reflects the trend line for fiscal year 2019 — plus the difference between fiscal year 2020 and fiscal year 2019.

This suggests that Title 42 expulsions delayed prospective migrants rather than deterred them — and they’re arriving now.

That would be consistent with nearly three decades of research in political science. Much of this research has been done since President Bill Clinton’s administration ran Operation Gatekeeper, which tried to keep out migrants by increasing funding and staff for border enforcement. Scholars consistently find that border security policies do not necessarily deter migration; rather, they delay migrants’ decisions to travel and change the routes they take.

Reassessing our understanding of undocumented immigration

So have Biden administration policies caused a crisis at the southern border? Evidence suggests not. The Trump administration oversaw a record in apprehensions in fiscal year 2019, before the pandemic shut the border. This year looks like the usual seasonal increase, plus migrants who would have come last year but could not.

Focusing on month-to-month differences in apprehensions is misleading; given seasonal patterns, each month should be considered in relation to the same month in previous years. Knowing those patterns, policymakers may be better able to plan, prepare and to manage the border.

Tom K. Wong (@tomwongphd) is associate professor and founding director of the U.S. Immigration Policy Center (USIPC) at the University of California at San Diego. His most recent book is “The Politics of Immigration: Partisanship, Demographic Change, and American National Identity” (Oxford University Press, 2017).

Gabriel De Roche (@gabederoche) is a PhD student in political science studying migration at the University of California at San Diego.

Jesus Rojas Venzor is a PhD student in political science at the University of California at San Diego.

Read more: