Washington  ­ To understand the complexities of migration you would have to listen to 1 billion stories - one from each of the people on the move across the globe.

What appears to be a mass movement is often a solitary enterprise of men, women and children, spurred individually by a common yearning to carve out better lives, moving on foot, by bus, rail or boat. Their journeys are often fraught with dangers and filled with hurdles that can make the passage take weeks, months or even years.

Migration impacts almost every country, and many are simultaneously countries of migrant origin, transit and destination.

More than ever, human mobility has become a life choice - usually driven by disparities in demography, income and employment opportunities.

But there are other factors that cause migration: conflicts, political and religious persecution, climate change, epidemics such as AIDS, and natural disasters such as earthquakes and tsunamis.

The United Nations Development Programme's Human Development Report 2009 estimates nearly 1 billion people - or one out of seven in the world - are migrants.

The United Nations Development Programme (UNDP) report challenges several misconceptions about migration and makes the argument that migration can contribute significantly to human development and encourages nations to open up and lower constraints to movement.

Migration can raise people's income, health and education prospects. "Most importantly, being able to decide where to live is a key element of human freedom," the report says.

The first possible surprise is that most migrants move within their home countries. An estimated 740 million people are internal migrants, while another 214 million people are international migrants.

The archetypical migrant is an impoverished person seeking opportunity in a wealthier country. But the UNDP report points out that while almost 70 million international migrants have moved from a developing to a developed country, more than twice that number move from one developing country to another or between relatively rich nations.

While the United States and more recently Europe continue to attract large numbers of migrants, new destination countries include Costa Rica, Morocco and Thailand.

In September, a rusty boat turned up crippled and adrift off Costa Rica's Limon province transporting an odd mix of 54 people from Nepal and several African countries. Three Colombian crew members were arrested as suspected smugglers.

Authorities in the region say it is further evidence of a growing trend of migrants from Africa and other regions trying to use Central America as a funnel into the United States and Canada.

In destination countries, migration is often viewed as a problem created by migrants.

Measures to combat illegal immigration in the United States have included physical barriers and high-tech monitoring built along the Mexican border.

European Union interior ministers called in July for strengthening of the EU border-control agency. With African boat people washing up on Mediterranean beaches and Middle Eastern migrants transiting through Eastern Europe, the EU has struggled to find a way to share the burden on its frontier members of stopping and deterring migrants.

Even India has built a fence along its entire border with Bangladesh to keep out migrant workers.

Contrary to common perceptions, the UNDP report found that migrants boost economic output and produce more than they cost.

In countries facing sharp declines in population in the coming decades, particularly Japan and much of Europe, demographers point to immigration as a potential source of population growth. New arrivals also tend to have larger families, shoring up the falling native birth rates at least in the first generation.

Migrants without legal residency or work permits are especially at risk both in transit and even after arrival in destination countries. Their illegal status makes them vulnerable to both criminals and unscrupulous employers who may expose them to undue workplace hazards or fail to deliver promised pay.

If they can weather the risks, migrants themselves typically benefit from higher incomes and better access to education and health ­ all benefiting their families left behind, too.

Poor countries realize major benefits of remittances from their workers abroad. Research shows that total remittance flows to developing countries are around four times the total official development aid they receive.

But the brain drain of educated people going abroad also takes a toll at home.

Amid the global recession and rising unemployment in most rich countries, migration rates are showing signs of slowing. In some places, migrants are being forcibly pushed out.

"The recession should be seized as an opportunity to instate a new deal of migrants - one that will benefit workers at home and abroad while guarding against a protectionist backlash," said Jeni Klugman, lead author of the UNDP report.

"With recovery, many of the same underlying trends that have been driving movement during the past half-century will resurface, attracting more people to move."

But not all migration is about work and economic opportunity.

In June, the United Nations refugee agency (UNHCR) reported that an estimated 42 million people were forced to flee their homes during 2008 and warned that the situation had become more dire since the beginning of 2009.

About 26 million were displaced within their own countries, while another 16 million became refugees and asylum-seekers who crossed international borders.

Handling refugees has become a political issue in some developed countries, including parts of the European Union.

The UNHCR, though, points out that 80 per cent of all refugees live in the developing world and has criticized wealthier nations for not offering enough support to humanitarian efforts. The agency has refuted claims of advanced economies being "flooded" with refugees.

Pakistan tops the list of crisis countries with 1.8 million refugees, but the situation has become dramatically worse since the government launched a major offensive in April against Taliban rebels.

Antonio Guterres, UN high commissioner for refugees, called Pakistan "probably the most challenging displacement crisis since the Rwanda crisis" in 1994.

Other countries hosting large numbers of refugees include Syria with 1.1 million and Iran with 980,000. Germany led Europe with 582,700 refugees.

Conflicts and disasters might be part of the human condition. But further dangers loom on the horizon.

At a United Nations climate conference earlier this year in Bonn, Germany, researchers said that by 2050, some 200 million people could be displaced and sent looking for new homes by drought, floods and similar disasters attributable to global warming.

One estimate at the conference had some 20 million people - mostly moving within their countries - were forced to become climate refugees in 2008 after climate change contributed to natural disasters. (dpa)