driving economic transformation. Historians L. H. Gann and Peter Duignan have argued that Africa probably benefited from colonialism on balance. Although it had its faults, colonialism was probably "one of the most efficacious engines for cultural diffusion in world history". These views, however, are controversial and are rejected by some who, on balance, see colonialism as bad. The economic historian David Kenneth Fieldhouse has taken a kind of middle position, arguing that the effects of colonialism were actually limited and their main weakness wasn't in deliberate underdevelopment but in what it failed to do. Niall Ferguson agrees with his last point, arguing that colonialism's main weaknesses were sins of omission. Analysis of the economies of African states finds that independent states such as Liberia and Ethiopia did not have better economic performance than their post-colonial counterparts. In particular the economic performance of former British colonies was better than both independent states and former French colonies. Africa's relative poverty predates colonialism. Jared Diamond argues in Guns, Germs, and Steel that Africa has always been poor due to a number of ecological factors affecting historical development. These factors include low population density, lack of domesticated livestock and plants and the north–south orientation of Africa's geography. However Diamond's theories have been criticized by some including James Morris Blaut as a form of environmental determinism. Historian John K. Thornton argues that sub-Saharan Africa was relatively wealthy and technologically advanced until at least the seventeenth century. Some scholars who believe that Africa was generally poorer than the rest of the world throughout its history make exceptions for certain parts of Africa. Acemoglue and Robinson, for example, argue that most of Africa has always been relatively poor, but "Aksum, Ghana, Songhay, Mali, [and] Great Zimbabwe ... were probably as developed as their contemporaries anywhere in the world." A number of people including Rodney and Joseph E. Inikori have argued that the poverty of Africa at the onset of the colonial period was principally due to the demographic loss associated with the slave trade as well as other related societal shifts. Others such as J. D. Fage and David Eltis have rejected this view. Language diversity African countries suffer from communication difficulties caused by language diversity. Greenberg's diversity index is the chance that two randomly selected people would have different mother tongues. Out of the most diverse 25 countries according to this index, 18 (72%) are African.