Human capital flight, more commonly referred to as "brain drain," is the large-scale emigration of a large group of individuals with technical skills or knowledge. The reasons usually include two aspects which respectively come from countries and individuals. 
In terms of countries, the reasons may be social environment (in source countries: lack of opportunities, political instability, economic depression, health risks; in host countries: rich opportunities, political stability and freedom, developed economy, better living conditions).

In terms of individual reasons, there are family influences (overseas relatives, and personal preference: preference for exploring, such as ambition for an improved career).

The table below reflects the trend and demonstrates the immigration flow of skilled Africans to industrialized countries. International Office for Migration (IOM) and European Commission for Africa (ECA) estimates.

A new report says Africa has lost a third of its skilled professionals in recent decades and it is costing the continent $4b dollars a year to replace them with expatriates from the West