Last month, the New York Times published a detailed article that describes how some migrant teenagers who crossed the U.S. border without their parents are now working in difficult jobs. Although it focuses on the treatment of children as workers, the most important question is: why aren’t these kids traveling with their parents? The answer is clear and obvious: U.S. anti-asylum policy prohibits their parents from coming, so the children come alone. The results are tragic and predictable.

Indeed, I explained and predicted this outcome (here, here, here, and here). You have to read through most of the Times reporting before you find the most important line:

Parents know that they would be turned away at the border or quickly deported, so they send their children…

Figure 1 tracks the history of child asylum border policy, and the share of Central American children coming without their parents. After the official end of the policy known as “family separation” in 2018—under which children were taken from parents who were prosecuted criminally—the share of children from the Northern Triangle coming without their parents fell from 38 percent in July 2018 to a low of 20 percent in July 2019. The Trump administration released families with their children into the United States, so they largely came to the border together.

In June 2019, however, the Trump administration announced it would send Central American kids with their parents back to Mexico under the Migrant Protection Protocols (MPP) or Remain in Mexico policy. On September 29, 2019, the Trump administration started sending all Central American families back to Mexico, while unaccompanied children would continue to be released to sponsors in the United States. The consequence of this change was quickly felt. Children who were sent back to Mexico with their families started to cross alone. By March 2020, 69 percent of Central American children were coming alone.

Then, in late March 2020, the Trump administration started expelling unaccompanied kids too. These expulsions briefly brought the unaccompanied share down. But Mexico refused to take them, so they had to be flown to Central America, which was too logistically difficult to do at scale, and the Trump administration began to release most of them again. By September 2020, only 36 percent were expelled, and a court order in late November required them all to be released. The share of unaccompanied children remained high as the government continued to expel parents with children.

In January 2021, the Mexican state of Tamaulipas where most families were crossing refused to accept back families with young children, effectively exempting them from the expulsion policy. As a result, the share of expelled Central American families fell from 96 percent in December 2020 to a low of 18 percent in July 2021, and the share of children coming alone fell from a high of 71 percent in December to a low of 40 percent in August 2021. Expelling fewer families saved children from coming alone.

But the Biden administration responded to Tamaulipas’s refusal to accept families by bussing and flying them to other parts of Mexico to expel them. This increased the expulsion rate back to as high as 73 percent. The consequence was that the unaccompanied share of Central American children eventually reached 83 percent. In 97 percent of the months when most families were expelled, most kids came by themselves. The only exception was one month in 2020 when kids were also being expelled.

If the Biden administration continues to expel parents traveling with kids, children will continue to come alone without their parents. This fact—which was well-established before President Biden assumed office—is the most important thing to understand about children traveling to the border alone. If they travel alone, they can stay. If they come with their parents, they and their parents have a high probability of being expelled. The logical reaction for desperate people is to send their kids alone.