The Trump administration announced its new immigration proposal last week. President Trump proposes legalizing “dreamers,” immigrants who were brought into the country as children, in exchange for funding the border wall and making restrictive changes to legal migration. The White House will demand $25 billion in funding for the border wall.

The assumption is that this will wipe the immigration slate clean, and dreamers will find a solution in exchange for increased border security. But the border security itself creates dreamers.

For much of the 20th century, Mexican migration was seasonal, especially in the West and Southwest, timed to the rhythms of the agricultural harvest or other temporary jobs. Beginning in the 1980s, however, seasonal migration from Mexico dropped, as data from the Pew Hispanic Research Center show. The increasing risk and cost of crossing the border meant that many Mexican migrants stayed in the U.S. instead of engaging in short-term, shuttle migration. This produced more long-term residency among Mexican migrants. It also produced changes in the demographic composition of immigration, since the only way for families to be together was for women and children to join the men now settled semi-permanently in the U.S.

I saw the same trend during the more than 30 years I have been conducting research in rural Guatemala. The first mass migration to the U.S. from Guatemala occurred in the 1980s, as people fled because of civil war and political persecution. When the civil war ended in 1996, out migration continued to soar for economic reasons.

This migration was mostly male. Young men would work for a stint picking crops on U.S. farms or working in slaughterhouses or other jobs where there was labor demand. The men would visit Guatemala at Christmas or during their town’s patron saint festival. After a while, they would return to Guatemala to start a family. They sometimes made short return trips to the U.S., but they didn’t intend to settle here permanently.

By the 2000s, however, this pattern changed. Men could no longer travel back and forth with relative ease. Costs had soared. In 1990, it cost perhaps $2,000 to make the journey to the U.S. from Central America. Migrants made the trip accompanied by a trusted guide from their local community. Now, it costs $10,000 to $12,000, and the migration routes through Mexico are controlled by organized crime cartels. The high cost and danger are the result of increased border security, including the wall we already have.

Now, more women from Guatemala’s villages are making the journey, often with their children, in order to reunite with their spouses. Shuttle migration is less common, due to the high cost and risk. When these migrants cross the border now, they are much more likely to stay.

Decades of data show that enhanced border security won’t stop migration. But it can change it, sometimes with unintended consequences.

Legal relief for dreamers is urgent. But don’t tie it to Trump’s wasteful and useless border wall. There won’t be any clean slate on immigration until its root causes are addressed, particularly economic conditions in the sending countries and the U.S. labor market demand that continues to draw migrants north.

Elizabeth Oglesby is associate professor of Latin American Studies and Geography at the University of Arizona and a Public Voices fellow with the OpEd Project.