Brain drain, also known as human capital flight, is a serious issue in many parts of the world, as skilled professionals seek out work abroad rather than returning to work in their home country. Many are driven away by high unemployment, but issues like political oppression, lack of religious freedom and simply not being able to earn a big enough paycheck also play a significant role in exacerbating brain drain. The phenomenon is not only a serious economic issue (both in that the country loses workers and the money it put into training them in college), but one that often puts the health and safety of the nation's citizens at risk, creating long-term and potentially disastrous results for countries with high brain drain rates spanning several decades.

Here we've compiled a list of some of the nations that have been hardest hit by brain drain in that past few years. While some are making progress in reversing the process, others are seeing numbers rise and citizens migrating in larger numbers every year. These nations, often those in the developing world, must make major economic and social changes if they hope to retain their best and most skilled workers over the long term.

Ethiopia: Ethiopia produces a large number of qualified professionals, especially in the medical field, but is experiencing one of the worst brain drains of any country in the world. Attracted by better prospects overseas and in other African nations and pushed out by political persecution, Ethiopia's best and brightest haven't been sticking around after graduation. A recent study presented at the National Symposium on Ethiopian Diasporas revealed some shocking numbers, with the country losing about 75% of its skilled professionals over the past ten years. This exodus of highly qualified professionals has had a huge impact on the country, leaving it with too few physicians, engineers and scientists to fill positions the country desperately needs to thrive economically.

Nigeria: Nigeria is another African nation that has suffered due to a massive brain drain. With much of Nigeria still essentially a developing nation with unreliable power and few resources, higher level science, engineering and medical professionals often find little to motivate them into staying, especially with job offers from the U.S. and European nations exerting a powerful pull. Since Nigeria's brutal civil war in late 60s, the country has bounced between military governments and dictatorships, pushing out between 11 and 17 million people. Today, over 2 million Nigerians live in the U.S. alone, and of these about 20,000 are doctors and over 10,000 are academics. That's a heavy loss for a nation that so desperately needs qualified professionals to rebuild and improve its own resources.

Kenya: High unemployment rates, lack of resources and other factors have made Kenya one of the top brain drain countries in Africa. With fewer than 30% of Kenyans who study overseas returning to work in Kenya, the nation is feeling the hurt of losing so many skilled professionals. The Kenyan Medical Association has warned that emigration of medical professionals may make it impossible to provide health care to the country's residents – and the situation is already pretty dire. As of 2002, the public sector medical field had only 600 doctors and 70 dentists available to treat over 28 million citizens. With somewhere between 500,000 and 1.8 million Kenyans working and living overseas, the country is trying desperately to find a way to lure some of these citizens back home where their skills are much needed.

South Africa: Years of unrest, high crime rates, AIDS and lack of jobs have combined to make South Africa's brain drain a serious problem. Over the past three years, the country has lost over 100,000 workers, with an additional 70% of skilled South Africans saying they are considering leaving the nation. Losing so many skilled workers has a ripple effect, with the loss of each skilled professional costing about 10 unskilled jobs. Currently, the country is working to not only keep residents from leaving once they've completed their training, but to also attract professionals from other nations to South African businesses. Though there is still a long way to go to make this a reality.

Iran: In 2006, the IMF ranked Iran the highest in brain drain among 90 countries (both developed and less developed), with over 180,000 people leaving each year due to a poor job market and oppressive social conditions. In fact, it is estimated that over 25% of Iranians with post-secondary degrees live and work abroad, adding up to a total of 4 million Iranians living overseas. While the outflux of Iranians was scoffed at in the early 80's by government officials, today Iran is doing more to keep their skilled professionals at home, creating several national foundations aimed at improving the conditions for students in the sciences and increasing the number of graduate programs. Why pay attention now? Iran's brain drain is estimated to cost the country over $50 billion each year in economic losses.

China: China has become a major player in global economics, but despite a rapidly growing job force, it is having trouble hanging on to qualified professionals. Many believe the reason lies in censorship and lack of freedoms (including the one child policy), but whatever the true cause, 70% of Chinese students who study overseas never return to their homeland. Since 1978, over 1 million Chinese students have headed to universities located abroad to get their degrees, yet fewer than 275,000 have returned. In fact, many foreign schools actually work to attract Chinese students, only exacerbating the problem for a nation in need of top scientists and researchers. The Chinese government has worked hard to reverse the trend, providing top-tier students in science and engineering with a large number of incentives, yet the numbers are only growing larger. By the end of this year, the nation could see well over 200,000 students leaving.

Mexico: Mexican immigration is a big issue in the U.S., with tens of thousands of illegal immigrants crossing U.S. borders each year. Yet a large portion of Mexican immigrants don't fit this stereotype and are wealthy, well-educated and enter the country quite legally. Mexico is seeing a huge brain drain as more middle- and upper-class citizens move abroad, many who hold higher degrees and work in professional fields like medicine and law. The biggest cause for this is unemployment, with hundreds of thousands of skilled workers unable to find work, though security issues also play a big role. The higher the level of education, the less likely workers are to stay in Mexico, with about 70% of Mexicans with a PhD coming to work in America. The brain drain is worst in the sciences, where 79% of students who come to study in America never return home.

Jamaica: In a trend common for Caribbean nations, Jamaica faces one of the biggest brain drains in the region. Over 80% of Jamaica's citizens who've obtained higher education live abroad. Most of this migration is due to lack of jobs, as there simply aren't enough to go around for young graduates from university programs. What makes this loss so striking isn't just the high percentage, but that Jamaican citizens who choose to work abroad must pay a remittance to the Jamaican government. Even with these additional fees, students aren't enticed into staying to work in Jamaica. While much attention has been paid to the issue in recent years, little has changed and Jamaica. Along with Haiti, Suriname, Guyana and Grenada are losing between 70-90% of their college-educated force each year.

Malaysia: The brain drain in Malaysia has been steadily worsening, with the World Bank projecting it to intensify over the next few years. Currently, two out of every ten Malaysians with higher education seeks employment elsewhere, accounting for about 305,000 immigrants in 2009. There are a number of factors that contribute to this mass emigration, including job opportunities, political corruption and lack of religious freedom. Malaysia made big economic strides in the 90s, but growth has been halved in the past decade, slowing from 7.2% to just 4.6%. Experts believe this is largely due to brain drain, and caution that the nation could see serious economic issues if it doesn't do something more to encourage professionals to work in their home country.