The definition and boundaries of ethnic groups depend on how people perceive themselves and others. Language, cultural characteristics, and common ancestry may be used as markers of ethnic identity or difference, but they do not always define groups of people. Thus, the people called Atuot and the much larger group called Nuer spoke essentially the same language, shared many cultural characteristics, and acknowledged a common ancestry, but each group defined itself and the other as different. Identifying ethnic groups in Sudan was made more complicated by the multifaceted character of internal divisions among Arabic-speaking Muslims, the largest population that might be considered a single ethnic group.
The distinction between Sudan's Muslim and non-Muslim people has been of considerable importance in the country's history and provides a preliminary ordering of the ethnic groups. It does not, however, correspond in any simple way to distinctions based on linguistic, cultural, or racial criteria nor to social or political solidarity. Ethnic group names commonly used in Sudan and by foreign analysts are not always used by the people themselves. That is particularly true for non-Arabs known by names coined by Arabs or by the British, who based the names on terms used by Arabs or others not of the group itself. Thus, the Dinka and the Nuer, the largest groups in southern Sudan, call themselves, respectively, Jieng and Naath.
Data as of June 1991