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The term caste refers to an extreme form of social differentiation in which the groups that constitute society are ranked in a rigid hierarchical scale. In all true caste systems, society as a whole is divided into a series of groups recruited by birth: marriage is generally restricted to members of the same caste, castes are in some way associated with occupational specializations, and the order of castes is linked to a moral order that dictates codes of appropriate behaviour for each caste. The most elaborate form of caste is found in Hindu India. Others with less elaborate systems include Sri Lanka, Pakistan, Bangladesh and Nepal.