Bad Science in Path Dependancy: The Slave Trade and African Literacy

Here is a link to a recent paper discussing the negative correlation between African literacy rates in the colonial and post-colonial times and the "slave export intensity" during the pre-colonial era. 

Cherokee Gothic rightly points out that this is an example of economic path-dependency, a concept familiar to mathematicians: for certain quantities, it isn't where you end up, but how you got there. 

Example: if one pegs their net worth at the value of the stock AAPL, and through some sorcery predicts every upturn and downturn in the stock price, liquidating at peaks and converting all cash to stock at local minima, in a year that person would have a considerable amount more money than the person who held the stock fixed, and much more than the person who made the opposite choices. The path taken to the end is what caused the discrepancy in wealth after a year. 

Path dependancy is a familiar trope in political theater and is, depending on the political and philosophical bent of the person, the reason for gender and race gaps in education, poverty, and incarceration.  It can be summarized (thanks to Scott E. Page) in the "old Bostonian jump roping rhyme"

I eat my peas with honey. I’ve done it all my life. It makes ’em taste quite funny, but it keeps them on the knife.

And so this recent paper attempts to gauge the level to which slave export has biased literacy rates in African countries over the proceeding centuries. The answer? According to the abstract: "a negative and signicant relationship between slave
export intensity before the colonial era and literacy rates during the colonial era."

Here's the data used to support that claim, buried in a plot in the supplementary material. This image plots literacy rate against some normalized quantity representing pre-colonial slave exports as a percentage of the extant population. Where's the trend? 

To me (and this is just me), this appears to be a classic case of oversimplification. If the author wrote this paper with the exact opposite conclusion, I would be equally swayed. What causes the four outlier groups in slave export to be there? Why is there seemingly no trend? Why did the author connect two clusters with a line and call it a trend?

Bad science, even in Path Dependancy

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