The border crisis of 2019 is winding down after a surge in the number of apprehensions at the southern border to the highest since 2007. In contrast to the early 2000s, when there was a spike in illegal crossings by Mexican laborers looking for work, most of those entering now are families and children seeking asylum from the broken societies of Honduras, Guatemala and El Salvador.

At the peak of the influx in May, some commentators were suggesting that an appreciable fraction of the entire population of those three Central American countries might end up living in the U.S. But then border apprehensions began to decline as precipitously as they had risen. By September, they were back to the levels of the previous year:

Why did this happen? To some degree, it’s seasonal; in the past, border crossings tended to peak in March, but recently the peak has come in May. That means that the wave of migration may start up again in the early months of 2020.

But President Donald Trump’s policies may also have been partly responsible for the drop. Mass detention of migrant families has been the most visible and divisive of those policies, generating widespread outrage at the poor conditions at detention facilities and leading some critics to liken them to concentration camps. But other Trump initiatives have probably had a bigger effect on migrant flows.

First, Trump made a number of major changes to the asylum process. In June, he issued a rule making most people ineligible for asylum if they passed through a third country on the way to the U.S. and failed to seek refuge in that country (the rule is being challenged in the courts, but it’s in effect for now). In a similar vein, Trump has reached agreements with the governments of Honduras, Guatemala and El Salvador stipulating that asylum seekers from each of those countries must try to get asylum in any of the countries they pass through before requesting it in the US.

In addition, Trump’s so-called Migrant Protection Protocols now require many asylum seekers to wait in Mexico while their claims are processed. The administration has also sped up adjudication of asylum requests, which means migrants don’t get to remain in the U.S. for as long while waiting for a court date. These and other changes have made it almost impossible for Central Americans to get asylum in the U.S., and much more difficult to live in the country while waiting for a claim to go through.

Finally, Trump bullied Mexico into making it harder for Central Americans to reach the U.S. border in the first place. In response to tariff threats, Mexico deployed troops to its southern border with Guatemala in order to stop migrant caravans from entering; it also beefed up its troops at the U.S. border. In a strange way, Trump has thus fulfilled one of his most notorious campaign promises: to make Mexico pay the cost of stopping Latin American migration.

So the American public (or at least those who determined the outcome of the Electoral College) got what it asked for — a president who was willing to brutalize poor desperate migrants and transform the U.S. into a much less welcoming country in exchange for a slight slowing of demographic change.

But in the long term, these policies won’t matter much. As researchers Michael Clemens and Jimmy Graham of the Center for Global Development outline in a recent blog post, migration pressure from Central America is destined to slacken a lot in the very near future.

The reason is falling fertility. Central Americans are having fewer kids:

As Clemens and Graham show, this has led to a plunge in the growth rate in the number of young adults in those countries. Many of them will need to stay home to take care of aging parents and take over family businesses, leaving fewer who want to leave for the U.S. Clemens and Graham argue that a similar drop in the growth rate of Mexico’s population of young adults was followed 13 years later by a steep decline in migration rates. If Central America follows a similar pattern, migration from El Salvador and Honduras will begin to slacken in about 2021, while Guatemala will follow soon after.

Clemens and Graham also argue that simply letting Central Americans enter the U.S. as guest workers would have done a lot to slow the flow of border-crossing. Though many asylum-seekers come with their children, parents might be willing to come alone temporarily if it gave them a chance to earn some money to send back to their families.

So Trump’s policies may have been a lot of effort - and done a lot of damage to the U.S.’s reputation as an open and humane country - for very little gain. Even those who want to prevent Central American immigration could have simply waited for the migration wave to end. Instead, the U.S. is left with a militarized border, resentful southern neighbors and a byzantine asylum system designed to reject people rather than give them protection.