"Mr. Obama, tear down this wall." (Sam Hodgson/Bloomberg News)

The Berlin Wall wasn't just an ugly symbol of Communist oppression in Eastern Europe and Russia. For the people who lived behind it, the wall's  practical significance was that it stopped them from leaving their homes and seeking a better life in the West.

Protesters in Berlin breached the wall 25 years ago -- on  Nov. 9, 1989 -- which might seem like a long time ago. But barriers to migration -- both legal and physical -- are still keeping people in developing countries poor, while posing a real, long-term risk to the economies of rich countries.

Several economists have made this point forcefully in the past few days. Tyler Cowen argues that in poor countries where the population is projected to grow enormously in the next few decades, the only way people will have enough to get by is if they can move to wealthier parts of the world. Meanwhile, as the fertility rate falls in the developed world and the population ages, those countries will have to rely on young migrants from elsewhere to avoid a lasting decline in their economic vitality.

The argument for open borders goes beyond self-interest. Tim Harford notes how unfair it is  that that someone's nationality is the most important factor in his or her chances in life. The differences between countries are much greater than the differences within them. McDonald's employees in the United States make 10 times what they would make doing the same jobs in India. As Eric Posner and Glen Weyl note, the poorest Americans -- those making $3,000 or $4,000 a year -- are doing better than three-in-five people around the world, at least in terms of dollars. A real commitment to fairness, Posner and Weyl argue, means giving people in poor countries an opportunity to move to richer ones.

Frustrated crowds looking for barriers made of cement and wire to topple won't always find them, but here are five places that could benefit from the crumbling of "walls" around the world.

  • Japan: Thecountry could use more liberal immigration policies probably more than any other developed nation, but an antipathy toward immigrations and the Pacific Ocean keep migrants out more effectively than any wall. Given those policies and the country's low birthrates, it's no surprise that the nation's economy has been near-stagnant for so long.
  • Australia: Theislandnation has also taken drastic measures to stop migrants from coming ashore. The country built a detention center in the heart of the jungle on Christmas Island, hundreds of miles northwest of the mainland at a cost of $370 million -- an enormously expensive project to detain around 1,200 people.
  • European Union:Migrants from all over Africa cross the Mediterranean to seek asylum in the EU on the Italian island of Lampedusa. Last year, the island made headlines when hundreds of people drowned after a overburdened boat bound for the island from the Libyan coast caught fire and capsized. Then again, there are still places where old-fashioned walls do matter. Take the Spanish enclave of Melilla in North Africa, where migrants climb fences to cross Europe's political boundary, or the eight-mile fence Greece built along its narrow border with Turkey.
  • United States: The U.S. maintains a southern fence of about 650 miles, even though just as many Mexicans are returning home as are immigrating. Tearing down this particular wall might be too bold a measure, but immigration reform could provide a nice boost to America's economy.

As the U.S. example shows, reforming immigration laws can be difficult politically. Posner and Weyl's unpleasant suggestion is that policymakers in rich countries seek a compromise with opponents of immigration. As Qatar does, they could allow in more workers without extending legal protections to them. It's an idea worth considering, at least, given how much there is to gain from immigration for everyone involved.

This is all easier said than done. China isn't eager for North Koreans to immigrate en masse across their border, even if Pyongyang would let them. In Israel and the Palestinian territories, concerns about economic freedom are an afterthought given the intensity and intricacy of the conflict there.

A border crisis was what finally forced Germany's two halves back together in 1989. East Germans were leaving illegally in droves through neighboring countries, and demonstrators pulled down the wall after East German officials fumbled their response. Following reunification, East Germans took advantage of their newfound freedom to move West by the millions -- a mass migration that continued until very recently. That's not counting all those in the rest of the Soviet bloc who left after the end of Communism, especially after some countries joined the European Union.

Per-capita income in East Germany has more than doubled since reunification, as The Economist reports. Almost every East German home now has central heating, compared to only three in five under Communism. Standards of living have improved in the West as well.

To be sure, those changes are largely due to the end of Soviet planning, not just to emigration from Eastern Germany. Still, the economic case for immigration is as strong today as it was in 1989. The easier it is to cross a border, the better off are the people who live on both sides of it.

Migrants cross into Spanish Melilla last month. (Jose Palazon Osma/AFP Photo/Prodein)