DAVID GREENE, HOST:
Whether they're going up or coming down, walls have been an important part of history.
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UNIDENTIFIED PERSON: It's one of the world's greatest engineering wonders - the Great Wall of China.
UNIDENTIFIED REPORTER #1: The Israeli government insists their separation wall is for security reasons - to stop Palestinians carrying out attacks in Israel.
UNIDENTIFIED REPORTER #2: From the west side of the Berlin Wall, the sound that you hear and what you're seeing tonight - hammers and chisels as young people take down this wall.
GREENE: That was in 1992, when the wall that separated communist East Germany with the Western world came down. Almost three decades on, the construction of another wall is being contemplated.
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PRESIDENT DONALD TRUMP: We will build a great wall along the southern border.
GREENE: We wanted to examine the human impact these kinds of barriers can have. Author Marcello Di Cintio has visited some of the world's most famous walls. And he told our colleague Noel King that they all have one thing in common.
MARCELLO DI CINTIO: They're theatrical, whether they were the Great Wall of China, whether they were Hadrian's Wall or, I would argue, the wall along the U.S.-Mexico border - is there to provide kind of an illusion of security, an illusion of strength - a kind of strength that does not actually bear out in reality.
NOEL KING, BYLINE: Are there examples of when a wall did what it was supposed to do, which is, ideally, keep some people out, keep other people in?
DI CINTIO: Yeah, it's funny. The lowest tech wall that I visited was the only one that did that. And that was in the Western Sahara, where the Moroccans had built, essentially, a sand berm in the desert battlefield where they were fighting against the Sahrawi rebels. And that one worked. It's a hump of sand. And it was meant to stop kind of rebel Jeeps from advancing on Moroccan soldiers. And it does just that.
KING: When you think about the potential for damaging effects on one side or the other, which walls come to mind?
DI CINTIO: Oh, there's so many. I mean, just to think about the walls that I found - their psychological effect. For example, I spent some time along the Indian-Bangladesh border. There's one area where the culture of the people on one side of the line was exactly the same as the culture on the other. They had the same language. They had the same religion. They ate the same food. They read the same poems. All of a sudden, India builds a, quote, unquote, "wall." And in this case, it was a three-strand barbed wire fence.
All of a sudden, people started talking about their relatives on the other side of the fence differently. People in India started - were telling me things like, you know, our relatives are becoming more Bangladeshi now, or, we can't trust them as much as we used to. And the only reason was that something about that - those three strands of barbed wire rendered the people on the other side their enemy.
KING: Well, yeah. What you're pointing to is that there's an actual psychological effect.
DI CINTIO: Well, you know - and that's nothing new, either. In East Berlin during the time of the Berlin Wall, there was a psychiatrist who noticed that his patients who lived within close physical proximity to the wall showed higher rates of psychological disorders, higher rates of depression. There was more alcoholism. There was more domestic abuse. And he blamed it on the wall. And he actually coined a term that meant wall disease in German. And he claimed that the only cure for this disease was for the wall to finally come down.
KING: But at the same time, aren't there some people who actually feel like the wall makes them safer? I mean, we talk about the wall in the U.S. a lot - people who support it as a measure of security, as something that's going to keep us safe.
DI CINTIO: But a sense of security is not the same as actual security. And I understand a sense of security is valuable. What I've seen is that what those walls claim to actually accomplish, they never actually accomplish it.
KING: Marcello Di Cintio is the author of "Walls: Travels Along The Barricades." Marcello, thanks so much.
DI CINTIO: Thank you. Transcript provided by NPR, Copyright NPR.