Has President Donald Trump, in his first six weeks in office, singlehandedly secured the US/Mexico border — simply by scaring people out of coming to the US?

It sounds absurd. But it’s plausible.

In the first two months of 2017, apprehensions of people crossing into the US from Mexico have fallen by more than half. In December, US Border Patrol agents caught 43,254 people trying to cross into the United States; in February, they caught 18,762.

The Trump administration is already bragging about its victory over the forces of illegality. Homeland Security Secretary John Kelly said in a statement that “the early results show that enforcement matters, deterrence matters, and that comprehensive immigration enforcement can make an impact.”

Kelly is almost certainly oversimplifying the causes for the sudden drop. But take a closer look at who, exactly, has stopped coming to the US since December, and it seems possible that he’s at least partly right.

Fewer people are coming to the US because fewer Central American children and families are coming to the US

The 18,762 people the US Border Patrol caught in February is the fewest they’ve caught in a single month in at least 17 years.

The Border Patrol’s job is to catch people trying to enter the US without papers between official entry points. For the past several years, those people have fallen into two distinct groups. Some are people trying to enter without getting caught, knowing that they’ll live as unauthorized immigrants in the US. Others are people trying to seek asylum in the US who, once they’re on US soil, often present themselves to Border Patrol agents to begin the asylum process.

The asylum seekers often come from the “Northern Triangle” of Central America (Guatemala, Honduras, and El Salvador), countries plagued by gang violence and murder rates you typically only see in war zones. Many of them are under 18, either traveling alone or with a parent as part of a family unit.

International law prevents the US from simply rejecting asylum seekers without giving them a chance to make their case, and US law puts strict restrictions on how immigration agents may treat children and families. They can’t simply be deported or turned away — which makes them an obstacle to any attempt to crack down on immigration across the board.

Over the past several years, there’s been a slight decline, after accounting for seasonal variation, in the number of adults trying to get into the US alone. But it’s been fairly consistent. What’s really driven big changes in apprehensions in recent years have been changes in the number of children and families coming to the US — the orange and red in this chart.

This isn’t the first time a big drop in children and families arriving in the US has led to a big drop in total apprehensions. The numbers dropped in the late summer and early fall of 2014, amid rising panic in the US about a “border crisis,” after the Obama administration promised to put more families in immigration detention. After rebounding, they dropped again at the beginning of 2016, as the Obama administration promised to track down Central American families who hadn’t shown up for their court hearings.

Deterrence might be working — but it might not last

The Trump administration’s border crackdown is, first and foremost, a crackdown on Central American children and families.

Trump’s executive order for border enforcement is still in the process of being turned into policy. But the federal government appears to be considering a broad range of options to dissuade people from seeking humanitarian relief here. Parents who come with their children might be separated from them — allowing the children to be treated as “unaccompanied minors,” who might be able to stay, while the parents are put in conventional immigration detention. Parents in the US who pay for their children to be brought here unaccompanied might be prosecuted and deported.

None of this has happened just yet. But that doesn’t mean it’s not already having an impact. The message is clear: The Trump administration intends it to be much harder for children and families coming to the US to stay here.

It’s extremely unlikely that Trump is the only reason for the drop. It takes several weeks for someone to come to the US from the Northern Triangle — not to mention the time it takes to save up to pay smugglers and coyotes — and smugglers and traffickers often spread misinformation to create fake demand for their services. And part of the reason the drop looks so big is because November and December 2016 were much busier than usual on the US/Mexico border — possibly because people were trying to make it to the US before Trump took office.

But Kelly may be right that the mere promise of a crackdown is preventing people from making the journey northward. It’s an important truth of immigration: People make rational decisions about whether it’s worth it to pick up their lives and try to make it somewhere else.

The statement from Kelly paints it as a victory that fewer children and families are making the “dangerous” journey to the US, an attitude the Obama administration shared. But the countries they’re staying in instead are also dangerous — perilously so. When someone is at risk of kidnapping, rape, or murder — or when her children are — it’s extremely hard to make staying where she is the more appealing option.

When families had to make decisions about whether to leave based on the theoretical possibility of a President Trump, it’s plausible they decided it would be a fate worse than life under gang warfare.

But as the Trump administration actually starts to implement its policies — policies that will either violate international law or allow persecuted asylum seekers to stay — that information will end up getting back to the children and families still in the Northern Triangle and weighing the risks. Deterrence might work, but fear is easier to instill than maintain.