Since the 1970s numerous historians have challenged Williams from various angles and Gad Heuman has concluded, "More recent research has rejected this conclusion; it is now clear that the colonies of the British Caribbean profited considerably during the Revolutionary and Napoleonic Wars." In his major attack on the Williams's thesis, Seymour Drescher argues that Britain's abolition of the slave trade in 1807 resulted not from the diminishing value of slavery for Britain but instead from the moral outrage of the British voting public. Critics have also argued that slavery remained profitable in the 1830s because of innovations in agriculture so the profit motive was not central to abolition. Richardson (1998) finds Williams's claims regarding the Industrial Revolution are exaggerated, for profits from the slave trade amounted to less than 1% of domestic investment in Britain. Richardson further challenges claims (by African scholars) that the slave trade caused widespread depopulation and economic distress in Africa—indeed that it caused the "underdevelopment" of Africa. Admitting the horrible suffering of slaves, he notes that many Africans benefited directly because the first stage of the trade was always firmly in the hands of Africans. European slave ships waited at ports to purchase cargoes of people who were captured in the hinterland by African dealers and tribal leaders. Richardson finds that the "terms of trade" (how much the ship owners paid for the slave cargo) moved heavily in favor of the Africans after about 1750. That is, indigenous elites inside West and Central Africa made large and growing profits from slavery, thus increasing their wealth and power.