Because the potential returns appear to be greater in poorer countries than in the developed world, modern economic theory implies that rich countries should continually invest in poor countries until returns balance out. In fact, this doesn’t happen. Economist Robert E. Lucas Jr. asks why in his groundbreaking 1990 article, “Why Doesn’t Capital Flow from Rich to Poor Countries?” The question has become known as the Lucas paradox. Lucas analyzes this, focusing especially on the role of human capital—the skill and experience that people bring to their work.

Lucas, a professor at the University of Chicago, won the Nobel Prize in Economics in 1995 for unrelated research nearly two decades earlier. But the Lucas paradox may be his lasting legacy, important not just to economists, but to anyone interested in understanding how economic development works in poorer countries.