During his State of the Union address, President Trump insisted he would successfully build a wall on the southern border that would block immigrants coming north through Mexico.

“Simply put,” the president said, “walls work, and walls save lives.”

Except some experts say otherwise.

First, while nations around the world are building border barriers at record rates, walls often simply deflect migrants to other, less-fortified points of entry. And second, studies show, undocumented and legal immigrants were less likely to commit crimes than people who were born here.

“Walls are not very effective at stopping movement. People can go around, over, under…,” University of Hawaii geography professor Reece Jones, author of Violent Borders: Refugees and the Right to Move, recently told the Inquirer.

The president claimed the Texas border city of El Paso once had one of the highest rates of violent crime and was considered one of country’s most dangerous places. But now, after a barrier was built, the president said, “El Paso is one of our safest cities.”

In fact, as Vox reporters point out, El Paso never ranked among the most dangerous cities. It had low rates of violent crime long before the government began to build a fence along the Rio Grande in 2008.

If El Paso is known for anything, it’s as a hub for the manufacture of custom, handmade cowboy boots, home to companies including Rocketbuster, J.B. Hill and Caboots.

Only about five border walls or fences stood around the globe at the end of World War II. Today there are about 70, with more planned or under construction.

The biggest danger of border walls, professor Jones said, is to people attempting to sneak into the country. A strong border defense meant to deter migrants generally only re-routes them, often to more dangerous and remote passages, Jones said.

“I’ll get it built,” the president insisted on Tuesday. “This is a smart, strategic, see-through steel barrier — not just a simple concrete wall. It will be deployed in the areas identified by border agents as having the greatest need, and as these agents will tell you, where walls go up, illegal crossings go way, way down.”

That’s in dispute too.

U.S. Customs and Border Protection, which spent about $2.3 billion to deploy fencing on the southwest border between 2007 and 2015, reported that the barriers supported agents’ ability to respond to illegal crossings, because the fences slowed the progress of migrants, according to a 2018 study by the U.S. Government Accountability Office. The CBP also took additional steps to build physical border infrastructure, including roads and lights.

However, the GAO said, the border patrol had not developed metrics to assess the contribution and impact of border fencing.