“SOMETHING there is,” runs a line from Robert Frost’s poem Mending Wall, “that doesn’t love a wall.” But for as long as mankind has been building, we have been building walls: around cities, along borders, across disputed lands; to protect, keep out, demarcate and divide.

Jericho, on what is now the West Bank, threw up its walls as early as 8000BC. China built stretches of its Great Wall by 700BC. Hadrian’s Wall, “to separate the Romans from the Barbarians”, came in AD129.

In recent times, France misplaced its faith in a supposedly impregnable barrier on its frontier with Germany. Three decades later, concrete and barbed wire was slicing Germany’s former capital in half as well. The Maginot Line did not work and the Berlin Wall did not last. But walls and fences have not stopped going up. Indeed, since the Iron Curtain came down a quarter of a century ago, the world has been busy building separation barriers at a rate perhaps unequalled in history: at least 6,000 miles of wire, concrete, steel, sand, stone, mesh; anything to keep peoples out - or in.

It is not just walls separating divided communities in cities such as Belfast and Homs, or compounds hermetically sealed to divide rich from poor such as in Sao Paulo. The vast majority of barriers are going up on borders - and not just around dictatorships or pariah states.

Most strikingly, some of the world’s leading democracies including the US, Israel and India have, in the past decade, built thousands of miles of barriers. Since 2006, the US has erected 600 miles of fence along its Mexican border. Israel is building a 400-mile West Bank barrier, plus another 165-mile fence along its Egyptian border. India has built a 340-mile barrier along the so-called Line of Control of its disputed border with Pakistan, and is busily constructing another 2,500-mile fence on its frontier with Bangladesh. Last year, Greece threw up a four-metre-high wall along its short land border with Turkey.

What is odd is that this building is happening at a time when less-physical walls appear to be crumbling. This is the age of the global economy, multinationals, vanishing trade barriers; of “the free movement of goods, capital, services and people”, unprecedented mobility and instantaneous communication.

So why build new walls - especially when, as history shows, the old ones rarely did what they set out to do. As Janet Napolitano, until recently US secretary of homeland security, once astutely observed: “Show me a 50ft wall, and I’ll show you a 51ft ladder.”

James Anderson, emeritus professor of political geography at Queen’s University Belfast, notes that walls get built for very different reasons. He says: “There are those built as a response to internal civil, often ethno-national, conflict, within states and often within cities. There are those erected because two groups are going at each other, but the state itself is not at stake - rich against poor, white against black, criminal against potential victim. And there are those that run along state borders.”

Justified more often than not, these days, as anti-terrorist measures, border fences are more likely to be aimed “at keeping out, or at least differentiating, migrant labour”, argues Anderson. He distinguishes, too, between walls that came from “the bottom up”, and those imposed from the top down.

Belfast’s walls, he notes, originated in 1969 as “defence mechanisms, barricades made of bedsteads and doors to stop vehicles coming in to your street”.

Thirty years on, they have become “part of people’s reality” and are still - perhaps uniquely - supported by almost all those who live beside them. Running for the most part parallel to the roads into the city centre, though, they are not “huge impediments” to day-to-day life.

By arrangement with the Guardian