“Dual Economy”

What is the “Dual Economy” referenced concerning Africa and in the aftermath of slavery?  Do you see this as an inclusionary or an extractionary system?

https://scholar.harvard.edu/jrobinson/publications/why-africa-poor

https://scholar.harvard.edu/files/nunn/files/empirical_slavery.pdf

https://www.youtube.com/watch?v=f_v-o5ryaUo