Last December, the Trump administration celebrated what it thought was a major accomplishment: a 24 percent drop in arrests at the U.S.-Mexico border in the 2017 fiscal year. The 310,531 arrests made, a 46-year low, marked a significant decrease in one of the key indicators that measures unauthorized crossings into the United States.

U.S. immigration officials attributed the decline to Trump’s hardline policies since taking office. “We have clearly seen the successful results of the president’s commitment to supporting the front-line officers and agents of [the Department of Homeland Security] as they enforce the law and secure our borders,” Elaine Duke, DHS’ acting secretary at the time, told reporters.

The celebration appears to be over. Customs and Border Protection statistics released earlier this week showed that arrests have risen for three straight months. While border crossings ebb and flow with the seasons, the numbers are markedly higher compared to the same period last year. U.S. officials made 160 percent more arrests in May 2018 than in May 2017, DHS officials said on Wednesday.

The DHS is putting a different spin on things now. These numbers show that while the Trump administration is restoring the rule of law, it will take a sustained effort and continuous commitment of resources over many months to disrupt cartels, smugglers, and nefarious actors,” spokesman Tyler Houlton said in a statement. “No one expects to reverse years of political inaction overnight or in a month.”

Border arrests are one of the figures by which Trump measures the success of his presidency. Since taking office, the president has also touted changes in crime and unemployment rates and the value of the stock market. But Trump has almost no direct control over the forces that shape these numbers. That makes his reliance on them something of a double-edged sword: taking credit for the positive results means taking responsibility for the negative ones as well.

Immigration experts acknowledged that Trump’s surprise election may have been responsible for the severe, historically aberrant drop in border arrests in 2017. At the same time, they pointed to larger economic and social forces as the primary factors that drive migration to the United States. Experts also indicated that the changing demographics of who crosses the border and why they attempt the arduous trek.

While Trump has often singled out Mexican migrants, whom he called “rapists” when he announced his campaign in 2015, they account for a shrinking proportion of U.S. border apprehensions. In 2000, roughly 98 percent of the 1.6 million migrants caught at the border hailed from Mexico. Both the overall number of migrants and the proportion from Mexico steadily dropped in the following years, especially after the Great Recession hit.

Doris Meissner, a senior fellow at the Migration Policy Institute, cited two factors for this broader decrease: changing demographics in Mexico that reduced the number of young people entering the workforce, and a post-9/11 surge in border-security funding that made it harder to enter the U.S. “Overall economic conditions have made it much more realistic for Mexicans to think of their future being in Mexico,” Meissner, who is also a former Immigration and Naturalization Service commissioner, told me. “Enforcement at the border has also been an important part of that. It’s gotten tighter and tighter.”

By 2017, only 48 percent of those arrested at the border were Mexican. The rest were mostly Guatemalans, Hondurans, and El Salvadorans seeking humanitarian asylum. “What has changed is that Mexicans have increasingly been replaced by Central Americans who are not so much following the seasonal rhythms of U.S. labor demand as trying to escape horrendous conditions at home,” Doug Massey, a Princeton University sociology professor who studies international migration, told me.

Unlike those migrating across the U.S. border for economic reasons, who are often young and male, asylum-seekers often arrive as families. The most common reason for fleeing to the United States from Central America is endemic gang violence, which drove homicide rates in El Salvador and Honduras to the world’s highest levels. Some of those gangs have their roots in large-scale U.S. deportations of asylum-seekers during the Reagan and Clinton administrations.

Earlier this year, Trump raged against the arrival of a caravan of Central American migrants as it journeyed towards the U.S. “The big Caravan of People from Honduras, now coming across Mexico and heading to our ‘Weak Laws’ Border, had better be stopped before it gets there,” he tweeted in April. Despite his threats, roughly 200 migrants presented themselves to immigration officials at a port of entry near San Diego later that month. Under federal law, they are required to prove they have a well-founded fear of persecution to qualify for asylum.

In May, Attorney General Jeff Sessions announced that the Justice Department would aim to prosecute 100 percent of unauthorized border crossings, which would include some asylum-seekers. This had a deterrence component to it: Because children can’t be held in jails, Sessions’s policy shift effectively mandated that they be separated from their parents and placed in the Department of Health and Human Services’ custody. Those separations can be a traumatic experience for parent and child alike.

Democrats criticized the policy as inhumane, and in a sign that even the White House realized it was indefensible, Trump falsely claimed the policy was required by federal law and blamed Democrats for it. But Sessions so far has stood his ground and shifted the blame onto those affected by his decision. “If people don’t want to be separated from their children, they should not bring them with them,” he told radio host Hugh Hewitt earlier this week.

It’s too early to tell whether the family-separation policy will actually act as a deterrent. If migrant numbers from Central America begin to decline in the coming months, that may indicate that the policy is discouraging would-be asylum-seekers from attempting to cross the border. But some immigration experts are skeptical that it would stop people who are often fleeing even greater cruelty.

“The forces that are driving them are more powerful than the deterrence policies being deployed,” Kevin Appleby, a senior policy director at the Center for Migration Studies, told me. “When you’re being threatened with your life [by gangs] within 24 hours, you’re not thinking about whether your kid will be taken away from you at the border.”