All of the forty largest global conurbations are migrant-rich whirlpools of human diversity. Taken together, those forty megacities have a bit less than 18 percent of the world's total population - but they generate two-thirds of the world's economic activity, and they're at the forefront of technology and science.

That's one of the striking findings cited in a new report presented on Monday in Berlin by Jochen Oltmer, a researcher at the University of Osnabrück's Institute for Research on Migration and Intercultural Studies (IMIS). The report, entitled "interrelationships between migration and development," was sponsored by two German non-government relief agencies, Welthungerhilfe and Terre des Hommes.

It's an indication of the powerful benefits migrant workers can bring to their new homelands. And the benefits flow in the other direction too: "Remittances of money from migrant workers back to their families in their countries of origin often go toward paying for education and health care, as well as consumer goods," said Oltmer.

"Workers returning to less-developed countries after a stint abroad can also bring new skills and capabilities back home with them, which benefits development," he added.

But migrant workers also face severe challenges - and so do their families back home. There are high social costs involved when someone spends years abroad, working under often very harsh conditions.

Migration downsides

Sita Ghimire, program director of the "Safer Migration Project" (SaMi) for the Swiss development agency Helveta in Nepal, said 29 percent of Nepal's GDP was funded by remittances from Nepali migrants working abroad - mostly in the Gulf Arab states or in Malaysia.

While that money can substantially improve the material conditions of the family members left behind in Nepal, "a family father might see his wife and kids for only a couple of weeks a year, for years on end," she said. "His children grow up without a dad at home."

For women, the social costs of migrant work can be very high as well. Ghimire said wives of Nepali migrant workers are often "blamed and shamed" if they have no husband at home, and thus subjected to hateful gossip and social exclusion.

Nepali women who work abroad - mostly as housekeepers in the homes of prosperous Gulf or Malaysian families - are extremely vulnerable to abuse.

"Housekeeping doesn't count as work under Gulf law," according to Ghimire. Women are trapped behind household walls with no one to turn to for support, "so there are no protections."

If a woman gets pregnant - in some cases because a male member of the household where she works has forced himself upon her - and she isn't legally married, then under Gulf law, she is put straight into prison, since extramarital sex is forbidden.

If she's legally married to someone back home, and can prove it, she won't be put into prison. But when she gets back home, her child may not be eligible for citizenship - and she's likely to face ostracism.

Bilateral agreements 'crucial'

Migrants don't just face loneliness and - in the case of women - the risk of sexual harassment. They're often required to work extremely hard for very low pay under abysmal safety conditions, for example in outdoor construction jobs in Gulf countries such as Qatar, where temperatures can reach 45 degrees C (113 degrees F).

Worse, employment brokers demand stiff fees to arrange for laborers to get jobs in the Gulf. Migrants often go deeply into debt, borrowing money at exorbitant interest rates to pay brokers, who are often dishonest about the pay and conditions awaiting workers abroad. Many migrants end up earning next to nothing on a net basis, sometimes for years.

Properly enforced bilateral agreements between migrant workers' countries of origin and host countries are crucial for the lives of migrant workers to improve, Ghimire and Oltmer said. Diaspora organizations set up by migrants who have successfully made their homes in a new country have a special role to play in lobbying for better rights and protections for migrants, and helping them settle in.

Refugees have it even harder

Refugees fleeing from war or natural disaster face even tougher challenges, since they can't go back home. Today, an estimated 60 million refugees are on the move. That's the highest number since World War II - and the number continues to grow, owing to disasters like the war in Syria.

"Governments need to recognize that refugee flows aren't a temporary problem to be managed until they go away again," said Danuta Sacher, CEO of the Osnabrück-based relief agency Terre des Hommes. "Migration is a global phenomenon that we must recognize as a permanent part of reality."

Of those estimated 60 million refugees in 2014, 39.2 million were "internally displaced persons" (IDPs) and 19.5 million were cross-border refugees. Half of all refugees were under the age of 18, and two-thirds under 25, Sacher said.

"Contrary to the perception in Europe that hordes of refugees are streaming across our borders, 86 percent of all refugees live in the Global South," Oltmer said. "Ten years ago, the number was 70 percent. As a percentage of the total, the North is actually taking in fewer refugees than a decade ago."

Oltmer, Sacher and Ghimire all called for Europe to develop a coherent set of policies governing the status of refugees and migrant workers and recognizing basic human and economic rights and protections.

The goal: To enable each individual to unfold their potential by gaining enough control over their own circumstances to live successful lives - regardless of where in the world the vagaries of fate take them.