Globalisation was supposed to tear down barriers, but security fears and a widespread refusal to help migrants and refugees have fuelled a new spate of wall-building across the world, even if experts doubt their long-term effectiveness.

When the Berlin Wall was torn down a quarter-century ago, there were 16 border fences around the world. Today, there are 65 either completed or under construction, according to Quebec University expert Elisabeth Vallet.

From Israel's separation barrier (or "apartheid wall" as it is known by the Palestinians), to the 2,500-mile (4,000-kilometre) barbed-wire fence India is building around Bangladesh, to the enormous sand "berm" that separates Morocco from rebel-held parts of the Western Sahara -- walls and fences are ever-more popular with politicians wanting to look tough on migration and security.

US presidential hopeful Donald Trump has made plans for a wall along the border with Mexico -- to keep out what he called "criminals, drug dealers, rapists" -- central to his inflammatory campaign.

In July, Hungary's right-wing government began building a four-metre-high (13 feet) fence along its border with Serbia to stanch the flow of refugees from Syria, Iraq and Afghanistan.

"We have only recently taken down walls in Europe; we should not be putting them up," was one EU spokesperson's exasperated response.

The illusion of security

But in spite of the aggressive symbolism, it is not clear that walls are truly effective.

"The one thing all these walls have in common is that their main function is theatre," said Marcello Di Cintio, author of "Walls: Travels Along the Barricades".

"You can't dismiss that illusion, it's important to people, but they provide the sense of security, not real security."

The limits of their effectiveness are visible everywhere.

Migrants still reach their Eldorados. Cocaine still reaches the coffee tables of Manhattan. The fearsome Berlin Wall with its trigger-happy sentries still leaked thousands of refugees even in its most forbidding years.

Supporters of walls say a few leaks are better than a flood. But, Di Cintio argues we must also consider the psychological price they exact.

He cites the Native American Tohono O'odham tribe, whose elders started to die off in apparent grief when the Mexican border fence cut them off from their ceremonial sites.

Their story carries shades of the "wall disease" diagnosed by Berlin psychologist Dietfried Muller-Hegemann in the 1970s after he found heightened levels of depression, alcoholism and domestic abuse among those living in the shadow of the barricade.

Di Cintio also talked to Bangladeshi farmers suddenly cut off from their neighbours when India erected the simple barbed-wire fence between them in the last decade. Within a few months, he said, they had started expressing distrust and dislike for "those people" on the other side.

"I was struck every time at how a structure so simple as a wall or fence can have these profound psychological effects," says Di Cintio.

Hitting the poor hardest

At a localised level, a wall offers more security than no wall.

But they do little to address the roots of insecurity and migration -- global asylum applications and terrorist attacks have risen hugely despite the flurry of wall-building. Rather, they just force groups to adapt.

They are mostly effective against the poorest and most desperate, says Reece Jones, a University of Hawaii professor and author of "Border Walls: Security and the War on Terror in the United States, India and Israel".

"Well-funded drug cartels and terrorist groups are not affected by walls at all because they have the resources to enter by safer methods, most likely using fake documents," he said.

Shutting down border crossings only "funnels immigrants to more dangerous routes through the deserts of the US southwest or on rickety boats across the Mediterranean. The substantial increase in deaths at borders is the predictable result," said Jones.

More than 40,000 people have died trying to migrate since 2000, the International Organisation for Migration said last year

Real border control comes only through the slow, exhaustive work of building ties and sharing information with other countries, says Emmanuel Brunet-Jailly, from Canada's University of Victoria.

"But with the intense flows of people we see today, walls are perhaps necessary for politicians. They tap into old myths about what borders should be -- the line in the sand -- which humans relate to," he tells AFP.

"It's a lot more difficult for people to accept that diplomatic cooperation and sharing databases are much more effective in the long term."