Every year, thousands of highly educated Africans, including many of the continent's brightest academics and scholars, leave the continent to pursue careers in the developed world.
For years, debate has centred on how Sub-Saharan Africa can retain its academic talent and stem the flow of this 'brain drain'. More recently, though, there has been a re-evaluation: maybe this exodus is not necessarily detrimental to Africa's development and might instead lead to capacity building in the continent.
In a world of ever-increasing connectivity, mobility and international collaboration, migration is not one way - knowledge and skills circulate. So how can Africa maximise this 'brain circulation' for development, and engage with the academics of the African diaspora?
In 2010/11, there were 2.9 million tertiary-educated migrants from Sub-Saharan Africa living in OECD (Organisation for Economic Co-operation and Development) countries. Some 450,000 had arrived there in the previous five years. This overall figure represented one in nine people with tertiary qualifications in Sub-Saharan Africa. 
But reliable figures for the numbers of academics and researchers from Sub-Saharan Africa working in the developed world are few and far between.
The first wave of emigration of academics from Sub-Saharan Africa occurred in the 1970s and 1980s, as funding for African universities was drastically reduced, partly because of pressure from organisations such as the World Bank to divert funds away from universities and into primary and secondary education.
"Many African universities suffered greatly because senior members of staff left in the 1970s and 1980s - the big African diaspora," says Johann Mouton, director of the African Doctoral Academy, a South Africa-based facility that provides preparatory training for prospective doctoral students across Africa.
"People estimate that tens of thousands of academics and scholars were lost from the continent. You lost a lot of senior academics, but you also lost actual doctoral programmes - many African universities simply do not offer PhD programmes in certain fields. In the past, people had to go to the United States, not simply because of other reasons, but also because there was not that offering."
Thus the legacy of this initial diaspora is still felt today - it led to a pattern of Africa's brightest scholars having no choice but to leave to pursue their academic careers.
It is estimated that around half of Africans who study overseas do not return.  This is often not only because of the temptation of higher salaries abroad, but also because the lack of infrastructure and facilities in their native countries restricts their ability to continue their research.
Designing degrees to retain PhDs
Emmanuel Kweyu, operations director at Strathmore University's @iLabAfrica, in Kenya, is well aware of the dangers of losing students overseas.
"What has happened in normal practice is that many of the students go and do a PhD out there and then they don't come back," he says. "So we are trying to arrange some kind of sandwich programme between universities to build capacity."
Such a PhD programme would, he says, be based largely in Africa. "They will probably spend some time in Europe but most of the time will be spent here. They will be able to continue the links with Skype and video conferencing."
Sandwich courses make it less likely that students will migrate after finishing their studies and increase capacity building via the partnerships between the institutions involved, says Kweyu.
Another way to retain researchers may be to create places for the brightest students to study without the need to travel overseas. One main driving force behind the African Union's decision to establish the Pan African University was to reduce the brain drain by creating such centres of excellence.
Benefits of emigration
While it is clear that Sub-Saharan African governments and institutions need to create conditions that encourage researchers to remain on the continent, the migration of African academics to institutions in the West also has many benefits.
Recently, Africa has seen an increasing number of multinational companies opening research hubs in Africa as the continent's economy grows. Many of the researchers they are looking to employ are those with skills attained at the best research institutes in the world.
For example, Nigerian Uyi Stewart, the chief scientist at IBM Research - Africa, in Kenya, obtained his PhD in the United States and now has returned to Africa to take up the role. Around 70 per cent of the lab's employees are African.
Stewart says of African researchers living overseas: "Our goal is to train them and bring them back, and let them be part of this massive wind of change that is going on in Africa and be enablers to this transformation."
Even those researchers who have migrated overseas and will never return are not necessarily lost to the continent.
Having African scholars based at top research institutions across the world presents opportunities for Africa, particularly as Western universities increasingly look for international collaborations.
Migration in this context can be seen as 'brain circulation', says Paul Tiyambe Zeleza, a Malawian now working in the United States whose research has led to the creation of the Carnegie African Diaspora Fellowship Program.
The programme, which launched early this year, is granting short-term fellowships to 100 African-born scholars in Canada and the United States to collaborate with universities in several African countries, including Ghana, Kenya, Nigeria, South Africa, Tanzania and Uganda.
Zeleza stresses the importance of brain circulation in creating "diaspora knowledge networks". These networks - through the connections made via collaboration, exchange and networking opportunities - "ultimately support capacity building and innovation in home and host countries".
With increasing speeds of internet connection across Africa, it has become easier to connect academics working outside Africa with those in African institutions.
The UNESCO-HP Brain Gain project works in 20 African and Middle Eastern higher education institutions, enabling researchers to collaborate in real time on research projects with those who have migrated away. Grid and cloud technology gives researchers in Africa access to remote computer power, research labs and scientific instruments.
Out of the vacuum
A perfect example of how brain circulation can support capacity building is the experience of Alan Christoffels, the interim director of the South African National Bioinformatics Institute at the University of the Western Cape.
Having obtained his PhD at the university, Christoffels chose to further his research overseas, being "a firm believer that once a student completes his or her studies they have to go out there and just build a network of people and expand their collaborations".
He started out working on an international project in the lab of a Nobel laureate in Singapore, then set up his own lab before returning to South Africa seven years later.
"What has been great is that all the links I built up for the seven years have stayed with me and I have retained one project with a colleague there which has also sparked new projects. And my students have gained from that," he says.
"I sent a student to Singapore and I have sent students to the United States. I think the concept that we recognise is that science does not happen in a vacuum and we need to transfer that to our students."
Clearly, governments and institutions across Sub-Saharan Africa must address the lack of infrastructure, opportunities and facilities that force academics to migrate.
But they must also recognise that, in an increasingly interconnected world, the academic diaspora should be seen not as a drain, but rather as a huge resource that should be fully engaged for the development of the continent.
This article is part of the Spotlight on Making higher education work for Africa.
 Organisation for Economic Co-operation and Development World migration in figures (OECD, 2013)
 African Union Establishment of the Pan African University (African Union, 2011)