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　　Slavery existed in a large number of past societies whose general characteristics are well-known. It was rare among primitive peoples, such as the hunter-gatherer societies, because for slavery to flourish, social differentiation or stratification was essential. Also essential was an economic surplus, for slaves were often consumption goods who themselves had to be maintained rather than productive assets who generated income for their owner. Surplus was also essential in slave systems where the owners expected economic gain from slave ownership.

　　Ordinarily there had to be a perceived labour shortage, for otherwise it is unlikely that most people would bother to acquire or to keep slaves. Free land, and more generally, open resources, were often a prerequisite for slavery; in most cases where there were no open resources, non-slaves could be found who would fulfill the same social functions at lower cost. Last, some centralized governmental institutions willing to enforce slave laws had to exist, or else the property aspects of slavery were likely to be chimerical. Most of these conditions had to be present in order for slavery to exist in a society; if they all were, until the abolition movement of the 19th century swept throughout most of the world, it was almost certain that slavery would be present. Although slavery existed almost everywhere, it seems to have been especially important in the development of two of the world‘s major civilizations, Western (including ancient Greece and Rome) and Islāmic.

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