The number of people United States authorities arrested crossing the country's southern border dropped by almost 40,000 last month, from more than 144,000 arrests in May to fewer than 105,000 arrests in June. The drop came after President Donald Trump used the threat of tariffs to pressure the Mexican government to stop migrants traveling northwards through its country. But experts were quick to note that the blistering summer months in Mexico have, in most past years, correlated with a seasonal drop in arrivals.
While the summer heat and Mexican enforcement both certainly played a role in the drop in arrests, the trouble determining to what degree each factor explains the decrease illuminates the greater difficulties experts have in interpreting migration patterns.
In the Trump era, Customs and Border Protection's monthly release of its enforcement data has become a major press event. The administration has used it as an opportunity to portray a "crisis" on the border; this month, the administration used the 28 percent drop in apprehensions to indicate success in its efforts to pressure Mexican authorities to apprehend more migrants. But experts say there's a problem with trying to understand migration patterns using monthly statistics: With so many variables at play, looking at short-term data can lead to overly simplistic conclusions.
For one thing, it’s rare for someone to immigrate in the same month they actually make the decision to do so. As Sarah Bermeo, a professor at Duke University and expert in Central American migration explained to me earlier this year, northbound migrants have often spent multiple years preparing for their journey. First, most people don't spontaneously make the decision to leave their home country; instead, deciding to emigrate can take months and years of agonizing. Then there's the matter of preparation. Travel through Mexico is perilous for Central Americans, who are targets for assault, robbery, and kidnapping in that country. Many Central Americans will pay smugglers who can guarantee them safe passage, but, with smugglers charging rates of around $10,000, saving up for the journey can take multiple years.
This means that any change in U.S. immigration or border policy—say, Trump's decision to separate families in 2018, or the administration's current policy of returning asylum seekers to Mexico—might not be revealed in Border Patrol's arrest numbers until months or even years after the policies are announced and implemented.
There's another problem: While Border Patrol's arrest numbers are often used as an estimate of how many people are trying to cross the border, the number of apprehensions is not a perfect measure of arrivals. It's difficult to tell how many people are managing to evade Border Patrol and cross the border undetected.
In recent years, the way migrants interact with Border Patrol has changed, creating a new challenge in interpreting migration data. During the last high-water mark in immigration, in the early 2000s, most undocumented border crossers were trying to make it into the country without being arrested. Now, however, a huge number of people arrested by Border Patrol are intentionally surrendering themselves to the agents with the intention of asking for asylum. This could be warping our perspective of how many people are crossing the border because it means an increase in arrests doesn't necessarily reflect an increase in border crossers.
That leads to another variable: What if new policies, like the administration's "Remain in Mexico" plan, have discouraged migrants from asking for asylum? If that's the case, more people might be trying to cross the border undetected—and succeeding.
Interpreting migration data is far from an abstract academic exercise. It's how the Trump administration, with its focus on deterrence-based immigration policy, determines what policies do and do not discourage immigration. Inaccurate or overly simplistic interpretations of cause and effect in migration patterns could lead to unrealistic or ineffective immigration policies.