Why do Southern states have lower judicial efficiency? Blame the Yankees?

Many scholars believe that former colonies by Western countries have corrupted judicial system because the legal systems imposed by colonizers are not compatible with local conditions.

Professor Daniel Berkowtiz and Karen Clay show that this is true within the United States too, as our lands were acquired from many countries of very different legal systems. They argue that Southern states, which were acquired from the hands of civil law countries (France, Spain, and Mexico) after American revolution, had highly developed legal systems around the time of legal transplantation (i.e., they were forced to conform to British/American common law system when they joined the Union), and this made them vulnerable to transplantation effects, the same ways African colonies did.

Empirically, they show that the chaos created by legal transplantations can still be seen after 200 years. These Southern states so far still have lower quality state courts, less competent judges, and higher corruption. The results hold even after control for slavery history.

They argue that it is the transplantation, not the superiority or inferiority of either system,  that causes the problem. In states that were acquired from French before American revolution (Illinois, Indiana, Michigan, Ohio, Wisconsin), as they  were very lightly populated at that time of acquisition, transformation to common law system created less problems.

But isn’t it possible that civil law system is inherently inferior? That the longer a region is under administration of civil law courts the more corrupted it will become? Although most African countries have corrupted legal system, it still seems to me that those colonized by British are at least less corrupt, as shown by Professor Andrei Shleifer's famous "Law and Finance" (PDF file) paper.

Initial Conditions, Institutional Dynamics and Economic Performance: Evidence from the American States  (PDF file)
Abstract:   Using state-level data from the United States, we find that differences in colonial legal institutions have affected the current quality of state legal institutions. These differences in colonial legal institutions arose because some states were settled by Great Britain, a common law country, and other states were settled by France, Spain, and Mexico, all civil law countries. To explain these findings, we develop a transplant-civil law hypothesis that highlights the disruption associated with large-scale legal transplantation and the possible relative inefficiencies of colonial civil law. We find strong support for the transplant-civil law hypothesis. Our results are robust to the inclusion of additional variables capturing climate, geography, initial population, resource endowments, state level rules, and legal environment. Given the 150-200 year gap between the initial conditions and the measures of the current quality of legal institutions, we provide indirect evidence on the persistence of legal institutions. We then use initial legal systems as a source of exogenous variation in current institutions for providing a series of estimates of their impact on current economic performance.